Thanks for all the replies. First the water heater is new and is only 15 amps and according to the specs it can only be expected to raise about 20-25 deg. The 20-30A heaters have a much higher temperature rise capability. The heater I was thinking about is like
http://www.amazon.com/Kats-13150-Aluminum-Circulating-Heater/dp/B000BQUUR6/ref=sr_1_2?ie=UTF8&qid=1431292938&sr=8-2&keywords=engine+block+water+heater
I believe the point about the heat rise to the cabin may be the best argument against this. I too wonder about the potability safety but figure something could be found.
I have no room for a bigger propane heater and the propane capacity is depressingly small. Any other ideas?
