With us having a 4” spring king size mattress, we’d be using sheets and comforter if anything. Sleeping bags really weren’t going to be an option. We have a built in diesel heater in the camper as well and put the wife on the side that has the vent.You will find that a Rumpl is very slippery and will slide off. Sleeping bags will be warmer.
I sleep on a cot with an air mattress. Seems to keep the cold at bay.I keep a down comforter (for a queen size bed) in my RTT most all of the time. I've used that down to the teens just fine. A lot depends on what insulation you will have underneath you: if you have mattress foam in your RTT that helps a lot. On really cold nights I will boil water and put it in a 32oz nalgene bottle (the kind with the top that clamps on, or the screw on will work also). I through that in the RTT under the quilt a hour before I got to bed and that's enough to take the chill off until my body can warm up the blanket. I've brought down top quilts with me as well 'just in case' but I've never needed to supplement the one down blanket: it seems to do the job very well.
If you're using a tent on the ground, be sure to have some form of insulation so the ground isn't sucking the heat away from you. Budget options here are a sheet of Reflectix or windshield shade in a pinch. When my boys were little I did a Mojave trail trip and we slept on an air mattress that did nothing for insinuation to the ground. the ground froze that night and so did we.. I'll never do air mattress again.
Down is not cheap, but it's loft is very efficient for the weight and it compresses down like a champ.
If you don't like the closed nature of a mummy sleeping bag, check out the top quilts that hammock campers use: some of them will unbutton to open up into a blanket. PM me if you need more info on this option.