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 ...
The other thing is the flexible duct is probably too long.
We had the same low air flow problem with our Class C. The duct on the longer runs were cut about 18 inches too long. So, the factory folks just left a lazy "U" shape in the middle of the duct.
I cut out the extra length and straightened the duct and the air flow increased.