I have heat in the cabover, but it still gets cold up there and I addressed it as follows:
The front window - (dont know if you have one) even though mine came with a storm window, I put a piece of foam between the two pieces of glass, then put another piece of foam on the inside between the glass and the blind
I put a piece of foam under the mattress
I put two pieces of foam under the escape hatch (i.e. under the screen - not between the screen and the hatch). I cut it large enough to wedge in place. I put wrapped duct tape around them so they would not break up when I pull it out. I created a handle also using the duct tape so I can easily remove it if needed (this also helps keep things cool in the summer)
I created my own storm windows out of acrylic, weather stripping and screen latches. That way I would still have light through the windows, but eliminated a large amount of air loss.