Colder weather camping

There are really two different questions here - keeping the RV free of damage and keeping you comfortable. With daytime temps in the 40s/50s and nighttime in the 20s there's really no risk of causing any damage to the RV no matter what you do. Maybe a frozen water supply hose but not much else assuming it warms back up well above freezing during the day.

For the usage you are describing use your electric heaters as long as you are hooked to shore power if you prefer them over the furnace (I know I do). Heated underbelly and all that are really for temps below freezing for prolonged periods.