My first comment is my popup from Flagstaff had a heated mattresses. On cold nights, that was sweet.
I would concur that with the air mattress(es) under the factory mattress. I think you could easily get 1-2" of material under the mattress, much more than that, especially if you leave sheets and blankets in place, could be a problem.
2020 Chevy Silverado 1500
Forest River Wildwood XLite 263BHXL