If you are really concerned about running an A/C in hot weather, then one of your primary questions about ANY RV will be whether it has a generator. As mentioned, many trailers do not come equipped with a generator and many of the portable types will not be suitable for running an A/C. Lou made a valid point about the cost of running a generator at Wal Mart vs. going to a campground. Campgrounds have quite a price range with some being very expensive, some being very inexpensive, and most somewhere in between. A copy of the Trailer Life Campground Directory will give you a better idea of how much campgrounds cost. It's arranged by state and includes both private and public campgrounds. Each entry provides the amenities provided, site sizes, type of roads (dirt, gravel, etc.), and the like. By, the way, I strongly suspect that most of us find a campground when it's really hot outside. In a trailer you won't be running the generator while driving so the interior of the trailer will be pretty warm at the end of the day. With a motorhome we can run the generator and A/C while driving down the road so that keeps the temperature reasonable during the day which in turn makes it easier to cool the walls and contents after driving all day in the heat.
I would suggest you start a criteria list with three columns. One column will be "must have" features (such as a generator), one will be "don't need" features, and the middle one will be features that you can live with or without. Your list will grow and change (use pencil!) as you look inside various RVs. For example, some RV bathrooms have an enclosed toilet while others have it out in the middle of the bathroom. You might not care one way or the other so that would go into the middle column. But, with young children, you might want at least a minimal bathtub (not generally available in RVs but a shower can be used to "hose off" a sick kid). Think about how you believe you'll live on the road and what you'll need or not need and that will lead you to your various criteria.
ArdraF