When we had out TT, we were in a seasonal campground with a permanent spot. One of our neighbors was 350+ and had a 23' Layton. She could fit into the bathroom to use the toilet but could not use the shower. She could also not fit into the dinette. The stairs did not seem to be a problem. Don'y know what they were rated for but they were just the standard ones that came with the camper. It was really hard to get around her if multiple people were in the TT. Her TT did not have any slides.
I would take the suggestion of "showing him around" the TT to get an idea of how things will work. If seems OK, then a weekend trip before you go for an extended time.