I have a Thor Chateau 31W, it has the full length slideout on the drivers side, my problem is that the duct work under the sink that goes under floor over to the other side to the bathroom and living ...
Where is your furnace in relation to the ducts? I suspect it is closer to the vents that have a lot of air flow, and not so close to the ones that have minimal flow. Air will flow towards the closer ducts.