I enjoy camping in the winter as much as I do in summer, the decrease in population is a big part of the reason. I've done a half a dozen trips in sub freezing temps without freeze up, but my duration was 3 to 4 days max. I set my thermostat at 60. Coldest I was in was teens to single digits overnight and highs in the 20's for 4 days. I wonder if a few more degrees on the thermostat would help keep your fresh water from freezing up.
Thinking out loud...
I wonder about setting up some sort of return that would allow you to transfer water from the hot water tank to the fresh water tank. A few gallons a day may do the trick.
I also wonder about setting up one of those instant hot water systems in an RV to alleviate line freeze up concerns. A buddy had one of these in his house to keep a cold corner from freezing up. Tankless Water Heater - Gothotwater.com
, not the tankless part but the the water tank version. I'm not recommending this piece of equipment, just the concept. It would require a sensor or timer to occasionally kick on and circulate hot water through the hot side and push cold backwards on the cold side into the water heater which would provide a slight increase to the cold side with some movement, which is always good.
Would need to get a schematic of the plumbing to see if this is feasible or not. Both of these improvements would allow you to stay unfrozen with propane and only a little electricity.