"Cooler" is not very specific. I have more than one strategy for different degrees of cold, evolved from winter tent camping experience in Michigan and Ontario.
Down into the 50's, my mattress is sufficiently well insulated that a down or down substitute comforter is adequate, particularly if the bed is shared so that body heat can be shared. A bit cooler, I throw a wool blanket on top of the comforter.
40's and cooler, I use an appropriately rated sleeping bag. I have one for 40 F, another for 20 F; any colder and I'm not going to be camping without a heat source. I also have lighter bags for tent camping in warmer weather, but don't use those in the RV, except to carry as comforters for guests.
On the ground, or on a cot, I've used a space blanket under me as a thermal barrier (really important if the ground is frozen). I haven't found a need for that with the bed in my RV, as it seems to be an adequate barrier to circulating cold air. Closed cell foam (like memory foam) can work as a barrier immediately underneath a sleeping bag.