Using a window unit A/C to pipe cool air into Camper Shell

In my young days I had a couple old campers. I wouldn't consider a 1/2 ton truck for any TC. Your gen should be fine with a 5000 ac. As mentioned, buy one and try it.
I have also camped with a cap. You may be able to find a used tall model cap. Maybe mount the gen on a front hitch bracket and find a way to do a clean install of the ac so it's not susceptible to rain.

DOOR