The longest I've seen was a 53' custom built. We were in a state park that sites were first come first serve, but one was coned off, a pull through. Later the rig showed up. It was built on a moving van chassis. and pulled by a single axle tractor.
It was a traveling preacher with 6 kids. The thing had 3 bunk slides, a kitchen slide a a queen slide.
Talking to him he said he learned the hard way, to find a lot in town drop the trailer and drive through the park.He had gone into a park once and found out he couldn't make the turns for the trees. Had to back out.
So he had asked the park to hold the pull through while he went and got the trailer.
