The cleanest way to add water to a finished van: hang it underneath. Undercarriage tanks bolt to the frame rails below the floor and spare-tire tanks take over the vacated spare-wheel position, so your interior layout does not change at all.

The trade-off is freezing. Below-floor water is exposed, so in genuine winter conditions you want a tank heater pad, a heated bay, or a drain-and-run-dry habit. In three-season and warm-climate use, undercarriage mounting is the highest-value real estate on the vehicle.

These are commonly run as a second tank alongside a wheel-well tank - fresh below, grey inside, or the reverse. If you moved the spare to a rear-door tire carrier, the spare-tire tank is close to free capacity.