Extra water capacity is one of those upgrades that seems minor until you’re three days into boondocking and you’re rationing showers because your fresh water tank is empty. A spare tire water tank is an elegant solution: it repurposes the rear spare tire carrier for a 10-20 gallon water tank. It’s out of the way, accessible, and doesn’t take up interior space.

What You’re Working With

Sprinter vans (all generations) come with a spare tire mounted on the rear door. It hangs from a carrier bracket bolted to the door frame. That same bracket can hold a custom water tank instead of or in addition to the spare tire.

You can do this a few ways:

Replace the spare tire entirely with a water tank (your spare is now a DIY fix or roadside service situation)
Mount a water tank next to the spare tire
Mount a tank that wraps partially around the spare, maximizing the space

Most of our clients go with option 1 (replace the spare) to keep weight distribution clean and avoid a bulky rear profile. You’re carrying water anyway, which is heavier than a tire, so you’re not losing much.

Tank Options

You need a custom tank made to fit your carrier. We typically use rotomolded plastic tanks with a width that’s close to the spare tire width and deep enough to hold 15-20 gallons. Some builds use stainless steel for durability, but plastic is lighter and adequate if you’re not high-contact with rough terrain.

Key specs for the tank:

Must fit within the carrier opening dimensions (roughly 8-10 inches deep, 24-28 inches wide, depending on your van model)
Needs a fill neck with cap (accessible from outside the van)
Needs intake and outlet ports that you can plumb to your fresh water system
Should have a drain plug for winterization or emergency dumping

Custom tanks run $400-700 depending on size and material. Pre-made RV water carriers designed for roof mounting sometimes get repurposed here with custom brackets, which can save money.

Tools and Materials You’ll Need

Socket and wrench set (metric)
Sealant and adhesive (marine-grade, like Sikaflex)
Stainless hardware and clamps
Water line (typically 1/2 inch vinyl hose for fresh water)
Self-tapping stainless bolts if you’re adding new mounting points
Plastic trim removal tools (saves paint)

Installation Steps

Step 1: Remove the spare tire. Open the rear door and disconnect the existing spare tire from its carrier. You’ll typically see 4-6 bolts going through the tire rim into the carrier bracket. Remove them. The tire assembly is heavy, so use help here if you have it.

Step 2: Assess your carrier. Look at the existing carrier bracket. Bolts are usually in the four corners or along a frame rail. You’ll use the same bolt holes for your tank mounting, which is convenient. Check that the bracket is structurally sound and not rusted or bent.

Step 3: Prepare the tank. Before mounting, test your tank if possible. Fill it with water and check for leaks. Verify that the fill neck is accessible and the outlet ports are positioned where you’ll plumb them.

Step 4: Mount the tank. Bolt the tank to the existing carrier bracket using the same holes where the spare tire was mounted. Use stainless bolts and washers. Get help; a full water tank is heavy and awkward to maneuver into position while bolting.

Step 5: Seal penetrations. Wherever you’ve drilled new holes or removed old bolts, seal the holes with marine-grade sealant. The door is outside, so water can get in through old bolt holes. Don’t skip this step.

Step 6: Plumb the tank. Run fresh water lines from the tank fill location to your interior fill point (usually somewhere accessible inside, near the kitchen). Run outlet lines from the tank to your water distribution system inside the van. Use appropriate hose clamps at every connection. Fresh water lines should have a shutoff valve near the tank or inside the van so you can isolate the tank if needed.

Make sure inlet and outlet ports are clearly labeled. Use different color hose or tape if helpful - red or blue for fresh water is common. Don’t confuse inlet with outlet when running lines.

Step 7: Secure all hoses. Hoses running through the van need to be secured so they don’t vibrate or rub. Use zip ties or adhesive clips. Route hoses away from sharp edges and heat sources. If lines run near the engine area, use insulated hose or add protection.

Step 8: Test the system. Fill the tank partially and test for leaks. Check all connections. Verify that water flows to your interior system. Check for drips underneath the van where gray water drains.

Design Considerations

The spare tire water tank is typically not heated. If you’re in cold climates, this matters - water in an external tank can freeze. You can add heat tape around the tank or plan to winterize (drain the tank) if you’re storing in freezing temperatures.

The tank adds weight to the rear of the van. A 20-gallon tank holds about 167 pounds of water. This shifts weight distribution and can affect handling slightly, especially on rough roads. It’s worth accounting for in your overall van balance, though the effect is usually minimal on larger platforms like Sprinters.

Appearance is a consideration. Some people like the utilitarian look of a visible water tank. Others don’t want it as a visual element. If appearance matters, you can partially wrap the tank with trim or a cover, though you want to maintain access to the fill cap and any cleaning ports.

Common Mistakes to Avoid

Don’t over-tighten bolts. Torque specs for plastic tanks are lower than you’d think. Over-torquing cracks the tank material or strips the threaded inserts. Hand-tight plus a quarter turn is usually right.

Don’t forget to support the weight properly. The bolts to the carrier should transfer weight to the frame, not rely on just the plastic tank to hold structural load. Verify the bracket design distributes weight correctly.

Don’t plumb without checking slopes. If your water outlet line slopes downward (even slightly) back toward the tank outlet, water can siphon out while you’re driving. Route lines so gravity can’t create siphoning.

Don’t skip winterization if you’re in cold climates. A full tank of frozen water is heavy and can crack if it expands. Empty the tank before extended cold storage.

Maintenance

Inspect the tank monthly for leaks, especially where it bolts to the carrier. Check hose connections for weeping. In winter, drain the tank if you’re not using the van for more than a week.

Once or twice a year, open the fill cap and visually inspect inside the tank. Algae growth is rare in fresh water systems but can happen if light gets in or the water sits for extended periods. A simple tank rinse solves it.

Is This Right for You?

A spare tire water tank makes sense if:

You boondock for multiple days at a time
You want extra water without consuming interior space
Your existing tank capacity is limiting your trips
You’re willing to winterize if you’re in cold climates

It doesn’t make sense if:

You mostly stay at RV parks with water hookups
You’re in extremely cold climates where freezing is a constant risk
You want to keep your spare tire as emergency backup

For most of our clients, it’s a solid upgrade. Extra water capacity means more time between refill trips and more comfortable living while boondocking.

Getting It Right

If you’re installing this yourself, take your time with bolting and sealing. If you’d rather have it done right, we can handle the installation as part of a larger conversion or as a retrofit to an existing van. Reach out if you want to discuss.

For more information on water systems, check out our complete guide to off-grid water systems for van builds.

Spare Tire Water Tank Install Guide