If you're a winter weather RVer, then sometimes the cold can be a wee bit unsettling. Climbing up in the cabover bed can be a genuine test of your endurance. But it doesn't have to be that way--you can improve the insulation factor of your sleeping loft easily, and fairly inexpensively.
Years back we learned about the insulation level between the bed deck and the outside skin of one our truck campers. Water damage rotted the supporting frame under the bed and we had to pull the bed deck out to make repairs. Insulation in that old rig was pretty much non-existent. We took care of that after we repaired the framework by installing new styrofoam board. Happily, most contemporary truck campers already have foam insulation in place, so yanking up the bed deck is probably unnecessary for most.
However, you may find cold air infiltration. It's a fact, moving on down the road can cause structural movement that can allow for air to find its way up into your sleeping zone. This may be more of an issue for metal sided truck campers than fiberglass, but even fiberglassers can improve their overall winter chill resistance.
First, clear the deck. Take the mattress out and head down to the bed platform. In most cases you'll find bare plywood, probably with no sealant between the joints. Put a layer of insulated aluminum foil (often sold in cut to length pieces at the hardware store) down over the entire deck. Duct tape it thoroughly where the pieces butt together to cut air infiltration.
Next, add a layer of good quality carpet pad from the carpet warehouse. Use the best you can afford--you'll thank yourself. On top of that, a colorful carpet "remnant" from the carpet folks, chosen to meet your interior decoration tastes. You've increased the cold transmission resistance factor greatly.
Got a big "window" upstairs? That same insulated aluminum foil can be custom cut and fit over the existing window. Yes, it will cut the transmission of light, but it will also keep the cold out in winter, and the heat out in summer. Some RVers pile extra pillows up over the windows to do the same trick, allowing the light to be let in at will by readjusting the bolster locations.
Finally, a custom cut piece of foam rubber, a couple inches thick, can be stuffed up into the overhead vent. Just make sure that it's easily removed in a hurry, as often that overhead vent serves as your fire escape hatch--don't fix it so you can't access it, and consider too, you might be a bit groggy with sleep when you need to make a quick exit.