I have a heated basement in my 97 Bounder. Under the passenger side nightstand, there is a factory installed vent from the basement to the bedroom, so that air blown into the basement has a return path.
You might consider a opening from a storage compartment (preferably the electrical compartment to the inside) that is only about 3" diameter. Have you seen the brass looking covers used in desktops to cover a hole cut for computer cables? You can buy those at Office Depot and other places. Basically you cut a 2" or 3" hole in the desk, then run the wiring through it. This cover has a small U shape cut out about 3/4" or 1" across, so it will hide the hole and wires are still going through a small section of that opening.
Anyway I ran my #12 extension cord through my air vent to the basement, then across that compartment to where the factory 30 amp cord goes out, and then to the 20 amp receptacle in the campground. I also had my 30 amp cord to run the other things in the RV, including 2 more electric heaters normally set on low heat output. This gave me a total of about 4 KW of electric heat, without overloading my 30 amp service.
When I spent the winter in Portland OR, the coldest nights only dropped to about 26F, so that was plenty of heat. By running electric heaters at 10 cents per KW, I got 80,000 Btu's of heat for about $2.20. Running the furnace would take a gallon of propane, or about $2.50 at the time. If I had a heat pump, the same 80,000 Btu's would only require $0.80.
Fred.