It hard to beat the efficiency of the installed Heating system (mine is 19,000 BTU), I can't imagine heating my home's hot water Baseboard with a 40 gallon hot water heater.
the typical 6 gallon Propane/Electric hot water heater uses 1400 watts in electric mode. Would be easier to just plug in a 1500 watt space heater.
They also make a 6 gallon hot water heater with a heat exchanger (off engine), for domestic water only:
Atwood RV Water Heater w/ Heat Exchange - Gas and Electric - Automatic Pilot - 8,800 Btu - 6 Gal
Item # AT96163