I've been well over 300 pounds, the lighter the trailer the less it help up my weight. A smaller work/play type RV would have a stronger floor. The biggest problem will probably be the shower and maybe the toilet, but he could use a CG bath house. I was about 319 pounds naked when I owned my Arctic Fox TT and had no issues. I had a large residential style shower, but I wasn't 400 pounds either, so don't know first hand. All my RV's had porcelain toilets. The Artic Fox floors felt very solid, my light weight toy hauler floors flex some under my 270 pound weight. My current trailer would not hold up to a 400 pounder, I know that.
