Not sure if you went ahead with your project, but I was looking for the same answer. I found the best route and there is no cutting involved. In the closet you will find a little removable panel. Once removed, you might see some radiant barrier and insulation. If you get some flexi-duct you could easily finagle it down and back under the closet floor towards the bottom middle of the RV cap. There is a pretty good size opening under the pin box. When you look at your pin box, you will see some wires coming out of this rather large opening. Not sure if Home Depot or Lowes sells a vent register that will fit perfectly, but if you get something in the ballpark, you can probably bend a flange to keep it in place. Additionally, clean the surrounding opening with some 91% isopropyl alcohol, let it dry, and secure the vent register with some high temp foil tape. Install some 1/4” hardware cloth over the opening to prevent critters from creeping up in there. Also put a good amount of fiberglass insulation between the ductwork and vent register to keep your wiring safe. I plan to run my ductwork soon, but debating on just removing the cheap closet flooring and replace it with something a little more substantial (cedar planking). I know eventually I’m hoing to remove all that cheap closet material (walls and all) , spray foam the cap, really insulate the front well, and try to deck the closet out with all cedar. So many moths be flyin’ into my rig at night. The hard part will be finding thin cedar wall board to keep the weight to a minimum.
