There are a couple solutions for a propane/electric water heater.
http://www.precisiontemp.com/pt_rvmd_m500.html is one that was designed for boats and acts as an on-demand water heater. It has a power exhaust (would have to cut a hole through the floor, but better than through the compartment door!). But it's much more expensive than the Raritan.
If you're boondocking frequently you would probably be running your generator a few hours a day. That should be enough to heat water.