Hi Katie,
You didn't say what type of rv you were renting...assume it is a motorhome.
We use toasty sleeping bag to stay warm in Winter. Zipping two together helps you stay warmer. It was interesting; two people in a sleeping bag was a lot more comfortable that one in a sleeping bag, plus its fun and comforting to cuddle on a cold night. A sleeping cap also keeps the head warm when its cold, and wear socks. If my head and feet are warm then its pretty comfy.
We use a blanket to cover the access to the cab area. Lots of heat lost through the windows in the cab. Made keeping the house part warm much easier.
We set the thermostat quite low...just high enough to make sure that the pipes don't freeze..45 to 50 degrees.
Nice part of traveling in a motorhome...if it gets too cold or can't keep the batteries charged, you can head to warmer weather or a campground with hookups. I would not worry too much. Just stay flexible. If it looks like to much snow or unusually cold, then go to plan B. Always nice to have a plan B.
Often rental units charge by the hour for use of the generator so its nice not to have to use one too much.
Have fun.