It may have to do with which slide out mechanism you TT uses. I can tell you from experience that with the Keystone cable system my floor is not supported.
Here is the excerpt from the Keystone manual which apples to all models. In practice some models may be OK but Keystone has made this blanket warning.
"DO NOT step on the slide out floor when the room is retracted. The slide out floor is not supported by the main floor and stepping on the slide out floor may cause structural dmamage to the slide room."
