Go to Home Depot or Lowes and get some aluminum foil insulating material and cut 14.5" pieces and tape or staple 2 pieces together to make insulation stuffers for each of your ceiling vents. This is inexpensive and works incredibly well at stopping heat loss and cold gain. All RV ceiling vents are standard 14.5" square. We even use them in the summer to keep out the heat. Don't leave home without them! Putting down throw rugs helps big time in the cold. Keeping slides in prevents big heat loss/cold gain. We don't like to use the space heater due to the noise so we use two or three 1,500 heaters and they do a great job keeping the rig warn down to 20 degrees. Crack any cabinet door where you have plumbing/pipes, crack your shower door/bathroom door as well. As said earlier turn on and keep on the water heater. We have camped many times in 20 degree temps both dry camping and with FHUs. Never aproblem or uncomfortable with the above per-cautions.
