The Big Bend area is not a 'resort' area. Some RV parks might have 'resort' in their name but that surely doesn't mean it's truly a resort.
For Big Bend with hookups, stay at Rio Grande Village. It's in a very scenic part of the park with easy hiking right from your RV. There's a small grocery but I'd recommend that you bring all your food with you. This grocery has the bare minimum, would be expensive and there's no guarantee you'd even find milk or break on some days. This is a very remote area. The grocery also has an attached laundry and a shower room.
September will still be warm.