Moab will still be warm in September but keep in mind Spring and Fall are the prime times in Moab as the weather is a bit nicer. Check Moab's visitor website to find out if there are any major events during the time you intend on visiting. If there is a major event, make sure you try and get reservations as spots can fill up quick and you may have trouble finding a spot. You def want electric at a minimum for AC usage as it can get hot but with temps starting to cool into Fall, you may not need to use the AC but better to have the option than not.
I've stayed at both Slick Rock Campground and Archview RV Resort.
Slick Rock has a lot more large trees with shade but the sites are somewhat tight and is all dirt. It was a little cheaper than Archview. IIRC, Slick Rock Campground is one of the older RV parks so that is why they have more tree coverage.
Archview is newer so the trees aren't as big but the sites are a bit wider and also all gravel/dirt. It is a nice place and people are friendly.
Both locations have full hook-ups available and are fine places to stay.