In our Class C, the water tank is under the floor, above the rear axle. The shower is above the axle and the vanity sink just aft of the axle. Water heater is several yards forward of all this, and on the opposite side of the coach.
I think I could tee into the hot water line to the vanity, to a valve (electric or manual...) and from the tee into the FW tank. We rarely boondock, but our sites are Water/Electric, no Sewer. Just thinking, the way we camp, I could run from that tee to a drain through the floor. Running about a pint of warm fresh water onto the ground a couple times a day isn't going to hurt a thing.