Plumbing is the system builders most often get wrong, because it has to be designed before the cabinets and it is the hardest thing to change afterwards. Sequence it like this.

1. Capacity - decide fresh and grey volume, then pick tanks that fit the chassis voids rather than the cabinet you imagined. 20-30 gallons fresh suits most full-timers.
2. Placement - wheel-well tanks keep weight low and interior space free; undercarriage tanks free even more space but freeze.
3. Hot water - a 3L-9L AC/DC boiler heats from 12V while driving and 110V on shore power.
4. Delivery - a folding shower, an in-floor pan, or an exterior shower port for rinsing gear.

Jump to Van Water Tanks, Van Water Heaters or Van Showers. Every tank here is moulded per platform - check Sprinter, Transit and ProMaster fitments.