A couple options on the water heater (I'm sure there are more);
Under the van on the driver side, directly below the city water connection, you should find a small brass drain valve. This valve has pressure against it whenever the water pump is "on". You can add a "quick connect" fitting to this valve (using hose clamp and adapters to make it work if you don't have the special crimp tool for the PEX tubing). Install the mating quick connect fitting to the hose on the Coleman water heater that Jage has recommended. You now can use your on-board water and heat it with the Coleman. It has been done at least a couple times with another small propane "instant" water heater and worked great.
If you don't want to mess with setting anything up, the FlatPlate heat exchanger works really good and is easy to install. You would need to operate your engine when you wanted hot water, but you'd be charging your batteries at the same time. If you go this route, make sure you order the new style exterior shower connector from SMB, it is a very nice unit. http://www.sagiv.com/main.asp?sel_nav1=88&cat=site
. If you go this route, you might want to make sure you also have SMB supply a mixing valve to prevent hot water scalding. They should now be using this as standard equipment with any FlatPlate installed.