Yep, once it is out of your hands, it's best to forget about it and let the new owners do what they will. Because they will do whatever they have a mind to do and nothing you say or do will change them.
I frequently contemplate buying a steel cargo trailer with no roof vent and no rear door, and building it out as a TT. I could have a vent on the rear wall and a mini split A/C. Order the trailer with some steel attachment points welded to the roof, and mount solar panels up there without having to puncture the roof.