I've made some changes to the A/C installation, with credit going to a lot of other folks who've "experimented" with ways to get the A/C working to the point where it will actually cool the trailer down as required. While I've not tested on one of our HOT days (typically in the 90's with 90% humidity) it did work much better with outside temps in the 80's.
I did three major changes:
1. Installed a switch for actually controlling the little exhaust fan as opposed to it being controlled based on ambient temps (it would come on even when it was only 70 degf outside).
2. I "boxed" the A/C outlet and insulated surrounding cabinetry using Owens Corning Foamular boards (because I could get them at Home Depot!)
3. I purchased online a new Dometic hinged refrigerator vent door and replaced the small, restrictive vent installed by Shasta.
Pics are below, here's hoping it continues to work when we do hit the summer highs (but I'm convinced it can;t be any worse than what came from the factory).