"lacofdfireman"......You don't have enough batteries and you're setting your thermostat to high at night. You have some options. I'm not sure where your batteries are located (under the step or in a bay), but many will add two more batteries on the front frame rail near the radiator.
We've camped at Yosemite once a year at Thanksgiving for the last 25 years with three different RV's. Our Class C had the same issue you're having until I added two more batteries. Our current coach has six and is not an issue. Even though we have the battery bank now, we still use a Buddy Heater, as described on here, during the day.
As a former LACO (SD) fire investigator, I never really felt comfortable running the Buddy Heater at night. If we were doing more boondocking and needed heat like you do, I would look into something like this
Olympian. I think it's safer, permanently mounts to a wall (hopefully near your refer) where you can "T" off a gas line and run it off of your propane tank. Camping World has been selling them for years and you can physically take a look at one before buying.
