also worth checking , there is a 16" long x 1" thick piece of sponge rubber foam that seals between return and supply right under the base of the unit at the start of the duct work. Both front and rear AC units had 80% of the foam pushed out and letting air from the supply right back into the return section. fairly easy fix while I was cleaning the coils off in the unit. Remove the sheet metal plenum cover and look down inside the return air side where the power wires are located. I would be willing to bet almost all of the 2 year old coaches have the foam pushed out. Its only glued in on one side with contact cement.
Mark & Melissa
2015 Redwood RL38 , Kodiak disk brakes , 17.5" tires , 8K Morryde IS