In my opinion, it is the lack of support from the floor when walking on it, more than anything else. The manual of my 11.5 foot 1988 Bigfoot says not to use the TC when the floor is not supported, due to potential cracking of the gel coat in the floor area. My gray and black tanks are in the rear overhang, so they are never supported, on or off the truck. The water tank is in the front of the TC, and has lots of support, on or off the truck.
Walking in the TC off the truck`is like walking on a trampoline. I think most TCs with basements allow use on their jacks. The walking floor is attached to the TC differently than those with basements.
