To the best of my knowledge, there are no 12VDC water heaters. There is not enough electricity in anything less than a humongous battery bank to heat water.
As far as I know, all RV water heaters are propane, diesel, and/or 120VAC. Any 12VDC connection is either for pumps or igniters (versus pilot lights or piezoelectric igniters).
I have heard of some tanked heaters that use a heat exchanger to help keep the water in the tank hot when the engine is running.
I have an outside shower unit that I'll be installing on MLP, but it will be mostly for rinsing off. Inside "facilities" will be limited to a head and a sink for washing up.
The 1972 M375 motorhome that was given to me has a (very small) tub in it. By my calculations, it'd take half the fresh water tank to put just a few inches in the tub. So, it'd only really be usable at campsites with full hook-ups. More than likely, the price of a full hook-up campsite would be more than using the showers at the campground. (At campgrounds we've visited, access to the shower facilities was include in the price of all campsite, including the 'primitive' sites.)
I'm not trying to discourage you but don't be surprised if you're less than satisfied with 'onboard' showering.
Be careful with those water pumps. The 100psi pump could very well overwhelm your motorhome's plumbing and the 60psi might do likewise. It's worth noting that most RV supply stores sell pressure limiters for use when a RV is connected to a tap. (Most well pumps run at 30 or 60 psi.)
