Many RV's have a air duct that runs under a couch towards the drivers compartment, and that is about 4" diameter, and will carry about 100 cubic feet per minute. I guess that you need one more vent going forward?
You should be able to buy a 4" wye at a hardware store, and connect into the existing air ducts. Then run something like flexible dryer hose to the location that you need more heat. I guess if there is a slide out in the way, it will not work all that well. The manufacture should have put a floor vent in the basement someplace, where there is little traffic, (to avoid people stepping on it, or dirt getting inside from your shoes). It can be under a chair or couch, the heat will end up in the living room somehow. . .
If you spend a lot of time at campgrounds with hookups, try running 2-3 small 1,500 watt electric heaters on low heat output. Just make sure that you are not plugging into a inverter output receptacle, as you might trip a fuse on the inverter.
So if you have some receptacles that run on the inverter, don't use them for a heater line.
Have fun camping!
Fred.