You would have to add valves to shut off circulation through the engine and radiator when using the water heater for heat. Also, check BTU output of heater, I don't know if it's capable of heating the RV. If you have a forced air furnace, I'd think it would be more efficient and less draining on the batteries when boondocking. A lot of work to keep your toes warm!
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI