Undermount Spare Water Tank

Why choose an undermount spare water tank

An undermount spare water tank uses the spare tire cavity or open frame space beneath a vehicle to carry water without consuming interior volume. For camper vans, overland trucks, and expedition trailers, this approach frees space for living areas, gear, and batteries while keeping heavy water mass low in the chassis. The lower center of gravity can improve handling and reduce sway compared with tall interior tanks. Outside mounting also isolates pump vibration and water slosh noise from the cabin. The tradeoffs are exposure to road debris, temperature swings, and potential clearance conflicts that must be managed with the right design.

Undermount placement most often supports fresh or gray water. Fresh tanks must use food grade materials and fittings, with a cleanable interior and smooth plumbing to maintain water quality. Gray tanks benefit from durable walls, odor control, and an easy to reach dump valve. Many vans replace the rear tire cradle with a tank that mirrors the spare shape envelope, paired with a rear door or swing out carrier to relocate the spare. Others use the passenger side frame bay when exhaust and driveshaft routing allow.

Platform fitment and capacity planning

Fitment varies by platform. Mercedes Sprinter, Ford Transit, and Ram ProMaster each have unique frame clearances, crossmembers, and exhaust paths. Wheelbase length influences available space and tank size, as does rear wheel drive versus front wheel drive packaging. Before any bracket is designed, check the path of suspension travel, driveshafts, brake lines, DEF hardware, mufflers, and emissions sensors. Leave service access for oxygen sensors and heat shielding.

Capacity should align with travel style and payload. Water weighs about 8.34 pounds per gallon. A 20 gallon tank adds roughly 167 pounds before hardware. Include brackets, skid plates, pumps, filters, and full plumbing in your payload math. Make sure the total remains under the vehicle gross weight rating and axle ratings. Tanks with internal baffles reduce surge that can stress mounts and affect handling on rough roads. Level sensors, whether float or capacitance, help manage water use and support accurate range planning.

Material and capacity choices

Common tank materials include rotationally molded polyethylene, aluminum, and stainless steel. Food grade polyethylene is popular for fresh water thanks to low weight, corrosion resistance, and cost. Stainless excels for durability and sanitation where budget allows. Aluminum can save weight but needs careful treatment to prevent corrosion and taste issues with potable water. For most vans, 10 to 25 gallons underbody strikes a balance between range and weight. Gray tanks can be similar volume to match sink and shower use. Choose threaded or spin welded fittings that allow service and secure sealing under vibration.

Mounting and safety considerations

Never drill critical frame rails or compromise crash structures. Use engineered brackets that pick up factory holes, crossmembers, or subframe attachment points. Include isolation bushings or rubber washers to damp vibration and reduce stress on the tank. A formed or boxed skid plate protects the tank from rocks and road debris and also shields valves and sensors. Keep the bottom plane at or above adjacent low points to preserve departure and breakover angles. Route wiring and plumbing away from hot exhaust parts and shield them from abrasion.

Plumbing, venting, and freeze protection

Proper plumbing builds around four essentials: a dedicated fill, an atmospheric vent, a supply to the pump, and a drain or service port. The vent line should rise continuously to reduce burping and support full fills. Use reinforced potable rated hose or PEX, with stainless clamps and support every few inches to prevent sagging. A strainer on the pump inlet and a carbon block filter on the cold line improve taste and protect fixtures. In cold climates, add self regulating heat pads on the tank, wrap lines with insulation, and consider routing sensitive sections inside the thermal envelope where possible. Drain points should be reachable without crawling far under the rig.

Water quality, monitoring, and service life

Fresh water safety starts with a tank rated for potable use and smooth interior surfaces that resist biofilm. Periodic sanitation is key. Many travelers flush the tank with a mild unscented bleach solution, then rinse and run water through all fixtures. Inline carbon cartridges improve taste and odor, while sediment filters catch particulates picked up from trail fills. For extended off grid trips, a multistage filter or UV purifier at the faucet offers another layer of protection.

Monitoring helps avoid dry pumps and unexpected shortages. Pressure switches protect many diaphragm pumps, but a dedicated level sensor and display adds accuracy. Smart systems can show percentage remaining and low level alerts. A manual sight tube is a simple backup and can double as a fill indicator. For gray tanks, vent routing and a water trap at the sink drain help control odors. A heater pad and insulation on gray tanks maintain flow in shoulder seasons where nights dip below freezing.

Noise, vibration, and user experience

Mounting a pump near the tank under the floor can quiet the cabin significantly. Isolator mounts and soft hose sections reduce transmitted hum. Check valves prevent backflow and reduce cycle chatter. Quick connect exterior shower ports let you rinse gear without bringing mud inside. If you plan to use an on demand water heater, ensure supply line sizing supports required flow rates and consider a small accumulator to smooth pump pulses.

Legal and environmental notes

Keep gray water out of sensitive areas and follow local regulations for disposal. Use biodegradable soaps and avoid dumping near waterways. If you cross borders or state lines frequently, know the rules for carrying water in public lands and at checkpoints. Maintain ground clearance that suits your terrain. If the tank occupies the spare tire location, ensure the relocated spare is secured and does not block taillights or license plates.

Maintenance and inspection rhythm

Undermount tanks live in a harsh environment. Schedule periodic inspections of brackets, hardware torque, strap condition, and skid plate fasteners. Look for abrasion on hoses, cracked insulation, or heat affected sections near the exhaust. Cycle valves and drains to keep them from seizing. Replace filters on time and flush the system after silty or unknown water sources. A few minutes under the rig can prevent a trip ending leak.

How OZK Customs approaches undermount systems

When you want a clean interior, steady handling, and real water range, an undermount spare water tank can be the right call. The details make the difference. Brackets must match your frame. Skid plates need the right shape and air gap. Lines should be protected, heated where needed, and easy to service. That is where a purpose built installation matters.

OZK Customs designs and installs complete water systems that match platform, payload, and trip goals. If you are mapping a full cabin layout, our team integrates tank location with cabinetry, power, and climate systems so everything works together. Explore our recreational vans for inspiration, see what a tailored interior looks like on a custom build van, or review our finance friendly platforms on mainstream vans.

We build in Fayetteville Arkansas and hand off every rig at our Adventure Point lounge so you leave confident in how it all works.

Ready to add capacity and keep the cabin open for living space? Tell us how far you roam and we will spec a safe, quiet, and serviceable undermount water system built for your routes.

Ready for a reliable, quiet, and trail safe water system that protects clearance and adds real range? OZK Customs designs and installs undermount spare water tanks with proper brackets, insulation, heat management, filtration, and monitoring. Tell us how you travel and we will spec, fabricate, and install a system that fits your platform and goals. Start your build plan today.