I was with my bil this spring at a local rv dealer. He was buying a popup, and questioned the salesman about a fifth wheel in the future. He showed us a new Crossroads that he told him would be ok for his F150. I asked the pin weight and he said around #1400.
When we left I explained to him the knowledge I have gained on this site. That is a "dry" weight, his truck is stickered at #1426, and there was NO way it would work. As you already have said the payload is over from the start, unless you havea hd payload/max tow. Even then I believe you would be pushing it.
I guess I would say look carefully at the numbers, and ignore what the sales person tells you.
Good luck.