One of the four screws holding the AC grill to the ceiling fell out and I removed the grill so I could fix it. I noticed that hot air from the "attic" could be sucked into the room return air; look at the size of the holes Jayco broke into the luan for the thermostat wire and the 120v wire. Since those holes are into the "suction" side of the return air, air from the hot attic is pulled in along with the room return air:
I also noticed that one of the two supply duct openings was not sealed; this means that cold air is being blown into the "attic":
I need all the cold air I can have; I sealed the openings with adhesive backed aluminum tape, the kind used for sealing fiberglass ducts.
There were also open joints in the construction of the return side framing and supply side framing and I sealed up those joints as well with the aluminum tape. Those with inadequate cooling might want to look for these issues.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch