Cold air from a register

Any ideas?

It would be pertinent to know the rating of the furnace. Unfortunately, typical low BTU RV furnaces have a tough time trying to keep a barely insulated box up to comfortable temps when the mercury dives.

Additionally, there have been many posters on this Forum that have inspected their ductwork to find collapsed and separated duct work. A good upgrade is to replace the corrugated plastic ducts with either insulated ducts or ducts that have a smooth inner surface. These are attempts to make the furnace more efficient but there is still the barrier of a poorly designed insulation scheme to overcome.

One better way to exist inside with more comfort is to throw more heat into the enclosure. Electric heaters, catalytic propane-fired appliances, and floor surface heat pads all will raise the comfort level. Covering windows with an insulative material is a typical first step for many.

For those that will remain stationary during the Winter, skirting is a good solution and if auxiliary heat in the form of heat lamps or incandescent lighting can be added underneath, comfort will improve a lot.

FWIW, I removed the usual RV furnace that was in my Starcraft in favor of a space heater that I've used in three prior RV's. It does not need electricity to operate, it's a gas ring with safety valve mounted inside a tin can. I have the combustion air and flue make-up air piped in from the outside. The heater is located inside a masonry enclosure I built and I distribute the warm air off of it with 12V fans. Once the trailer and everything inside is heated, the control valve is set to "pilot" or the most minimum setting. I've used this appliance at -25f and it has heated a trailer space well. The Starcraft I now have is super-insulated with all wall, ceiling, and floor surfaces having a thermal break from frame and exterior siding. With that and new Low-e multipane windows, it is easy to heat and cool. Hopefully, some mfgs will get a clue and make a trailer that has some of the above upgrades so that an RV can truly be called "Four Season".