From a 5er experience, I can't say. But from a TT experience, actually the longer trailers (and mine is 35.5 feet long) are easier to back up. Not sure how that translates with a 5er.
But? When IS a tow vehicle-trailer combination TOO long? The answer rests with your state requirement for the total max length of your combined vehicle length. In Indiana, that's 60 feet! I'm not very far from that limit myself.
Now about campsites, yes, you won't fit in some. But, we've always found a spot available every campground we wanted to visit. It might not have been under a thicket of trees so deep you can't see the end of your camper, but we've always been able to get a spot somewhere. Maybe some campgrounds cannot accommodate longer trailers, that's why you always need to call ahead, and make reservations whenever possible. If boomdocking somewhere, length usually does not matter. A large parking lot, a road side rest area? All can accommodate 18 wheelers, so they will accommodate you too.
And remember, with a 5er, not only do you have to constantly be aware of length, more important, you have to be conscious of the height!