I found the same problem in 3 cabinets on our new 07. There is a difference in when the coach was produced. The early ones had some wood used as a plenum and the new ones have metal plenums. The first thing I did was seal any air leaks with metal foil tape, NOT DUCT TAPE. This helped a lot but I found that the metal plenums were still radiating a lot of heat. The plenums were then insulated and now the problem is very small. I plan to clean up the runs on the ducts and then insulate some of them, when I get home.
2007 Limited SE 40fdts
Las Vegas, NV