There was a man with an 18sq disassembled and carried on top of a large pop-up at Spring Fever for several years.

I had a long discussion with him about it, because I was considering it also.

He explained that most pop-ups have very little reserve carrying capacity (small frame and springs).
He found one model which had a large external storage box and plenty of weight capacity.

I have considered buying a small pop-up and removing it from the factory frame and
installing it on a nice long and strong boat trailer frame.

I saw a custom rig on ebay where they had put a small pop-up sideways on a long trailer with a flat deack for four-wheelers.