On a MUCH smaller scale, we just moved from our 23' HTT to a 26+' TT. Towing with an F-150 CC with factory tow, 5.4. Of the places we've camped over the 7+ seasons we had our HTT, I can think of only 1 that would accommodate something as large as a 39'. Even the small additional space we are at now, meant our fav site at our fav CG is a real PITA to get into, when it used to be a snap. We never expected to move from our HTT to the TT, but old age crept up when we weren't looking and this is much easier to setup. We love it and it's really perfect for the 2 of us, but allows for occasional guests with a sofa bed with air mattress. This really is our last TT.
My dear cousin started with an Outback, than a small A, then a bigger A, then a much bigger A and now a 45' monster A. He's happy with his status in life. His DW is miserable because they can't go much of anywhere and really CAMP. I don't consider KOA or similar "camping" and she agrees. Not only is the length a problem, but height with overhanging trees, as mentioned earlier. No narrow, winding roads to get to some CG with lots of trees and great views. We're having a family reunion this summer at LBL and there are sites that he COULD get this monster into, but not maneuver the CG road to get to the site, so they're several miles away at KOA.
Just from reading similar posts, it seems like the 32 - 35' range allows for most of the amenities wanted, plenty of interior space and still lets you get there from here without a problem. I could see the 39'+ a tow vehicle as being a bit tricky in even a Pilot, Flying J, etc that are set up for larger vehicles. That said, it really depends on how you camp and what your needs are.