Don't know about your hot water idea, you will still need a cold water supply, and a 40 gallon tank of 180 degree water will be one big heat source. Try a 40 gallon plastic tank(try RV salvage) with a water heater, mine has this one
ATWOOD MOBILE PRODUCTS | EHM6 WATER HEATER 6 Gallon, 110V, with heat exchanger | 93891 from the ShipStore.com on-line catalog. . you have more than enough hot water for 2 people to shower and still have hot water for the sink. If you want to shower after parking all night you can, the heater holds heat very well.
Custom sleeper are insulated very good, including the floor, then all pluming is inside the box, only the drains are exposed to the outside, don't forget to put in drain valves to bleed the water system, the water heater comes with one.