I've installed a portable AC/Dehumidifier (roll around type) in the 345. I cut an air exhaust vent hole in the floor under the couch, and use a dryer vent outlet flapper valve in the ducting system. Stainless screen mesh at the outlet prevents inscets and critters from coming into the coach. I also tied the water drain into the existing AC water drain.
Window air exhaust is feasible, but not as easily rigged up/rigged down when you want to use the unit, also adds one more rain leak possibility.
In the winter, you can simply exhaust the hot air into the cabin, adding heat and helping with the freezing problem.
Of course, I'm assuming you have 110 available, and, like you said, a cover sure helps.
"Suck it up, spend the bucks, do it right the first time."
WBCCI # 1113
Trailer '78 31' Sovereign
Living Large at an Airstream Park on the Largest Lake Totally Contained in Texas
Texas Airstream Harbor, Inc.