It all depends on what sort of camping you intend to do. For "RV Park" camping, more often than not you can get rigs in there up to or over 40 feet, and won't necessarily have to fit the Tow vehicle and trailer in the same spot. These are the sorts of places with dump stations or hook ups, and that are listed in places like RVParkReviews etc.
For Forest service and BLM type camping, my current rig at a little over 35 foot listed is to big for at least 65 percent or more of the campgrounds I'm aware of in Utah. They're the sorts of campgrounds with long dirt roads to get there, maybe a pit toilet and if you're really lucky a source of water somewhere in the campground. Of course all the best campsites I'm aware of involve getting there via backpack. In the end, it's all a mater of compromise and which style of camping you, and more importantly the spouse prefer.