1) An RV furnace will keep an RV quite warm down to the mid-20s without any problem. Especially a smaller unit with four or five people in it. A couple electric blankets or those small throws with electric heat might be very useful.
2) However, you must have electricity to use the furnace all night. A furnace blower will drain the batteries flat in a few hours. NPS campgrounds are out if the temps are near freezing.
3) You will find no rental RVs with solar power sufficient for the type of NPS campgrounds you want to use during that time of year. Solar takes a certain level of understanding to work well - and folks renting RVs don't want to spend dozens of hours learning how to make solar work best in an RV. It is also an expense the rental companies won't see a return upon.
Just a suggestion - if the wife and kids have never spent time in moderately high altitudes and the high desert night-time low temps - try to stay in commercial RV parks - if you can find some open.
The worst thing that can happen in my opinion, is for your family to be cold, damp and miserable on the trip. It can turn them off to camping permanently, or for a long time.
Make this trip as comfortable as possible - then branch out into some more adventurous camping on the next trip.
Let them be awed by the majesty of nature and the beauty - not shivering. Hook them on the good aspects of camping - then later teach them the minor inconveniences of less utility dependent camping necessary to see even greater sights/ areas.