Aqua-Hot is point-of-use (on-demand) system that uses the coach's diesel fuel ignited in a burner to heat the water. It has multiple water lines running to different locations in the coach (under floor) to provide room heating and hot water to all your fixtures.
http://www.aquahot.com/products/rv.aspx
It would not be easy to add this system to a coach that did not already have it. If you just wanted the water heating function, you could probably install the Aqua-Hot unit (If you have a compartment big enough. This unit is not supposed to be in shared space) and run your fuel lines to it, then tie in one electrical switch to turn it on/off. Then run your water lines for bath and sinks to it. I'm just not sure its worth that much effort.
Check out their website - maybe they have a similar unit more designed for what your wanting to do.