You can leave the heat on while you travel during the day, too, set to a low temperature so it doesn't use too much propane. If you're unwilling to travel with the propane on, you could stop every few hours to check on the temperature inside and use the furnace for a bit then if needed to warm things up sufficiently.
The latitude doesn't matter so much as the weather conditions. Since you're starting with a warm RV, I would guess that at least mid to high 20's during the day should be okay, and maybe even somewhat cooler, but a lot depends on the construction of your RV and how warm it starts out and various other variables.