We have spent two winters in Maine with just a 20 amp service. We had no electric heaters but we did have a good furnace. We planned ahead for the winter by banking up around the motorhome so no wind would get under it. WE also put cover over the windshield. When it snowed, we used a snowblower to blow the snow onto the banking around the motorhome. The blown snow helped keep the heat in. In preparation for stay the winter in the motorhome, we had the propane man install an external tank that he filled every 2 or 3 week. Keep the slides in so don't have to heat the extra space.
Get the furnace fixed...don't depend on electric heaters.
We had only one problem and that was with the humidity fed by the propane all over the windows. We kept they dry and no mold or other problems,
We also used a blue potty tank to dump black and gray water when necessary.
Stay warm.