Mine are designed to be used when in. The floor of each slideout is supported by rollers. They do compress the carpet, however.
Other slideout designs may not be designed to crry loads with the slideouts closed. Manufacturers of towable RVs tend to assume that they will not be occupied when moving. If this is a requiremnt for you, then you need to check it out for each unit you consider buying, and make the lack of slideout usability a "no sale" point.
You could also look at TTs without slideouts. They might be just as usable, and for something as big as a dinette+sofa slide, not having the slide might make the trailer 500-800 pounds lighter. Or maybe less, if the TT manufacturer is using flimsy mechanisms that can't support weight when in.
