Hang an extra blanket over the opening to the cab. The cab has little or no insulation.
Be sure your batteries are fully charged before you go to bed. The furnace uses about 7 amps. Ours runs about 1/3 to 1/2 of the time keeping the coach at 70 when it's 30 outside.
When recharging, it takes several hours to get a full charge. After only a hour the convenience panel will show four lights after the generator has been turned off, but this is misleading. The shows a "surface charge". It takes longer for the battery plates to become charged throughout.
2011 Sunseeker 3100
1997 Pathfinder Toad