Don,
I would be looking at a propane heater that doesn't use electricity. If you could couple that with an occasional 15 amp source for electric heat and running your space heaters for the tanks/pipes you'll be warm and happy.
I've camped at those temps using a vented propane space heater and it isn't hard to be comfortable. I believe the most propane we used was at 0f-->-15f for a week. IIRC, we burned 40-50lbs. This was in a less than stellar rig with insulation issues.
If you can use skirting, results are a lot better. If you insulate the under belly area real good, results are better. If you tear apart the whole rig and update the insulation, results are the best.;)