Options for in cabin heaters
If you have propane I second the Force 10. If not try either the Force 10 deisel heater jetted for kerosene and burning synthetic fuel or the Origo Alcohol heater, which puts out a bit less BTU. Either way electric from battery would be out of the question. If you already have a stove on board you can heat pots of water and leave them on the stoive overnight. Doesn''t heat it but sure take the chill off. Kerosene lamps also give off quite a bit of heat while lit.
P323 on LI Sound