Glad I wasn't the only one that could not stop thinking of the toilet and small bathroom.
No, no way 400 lbs makes a job in a small camper bathroom, and unlikely to get a shower either.
Do you weld? Cross members could be put under the walking-area of the floor by just welding in some extra pieces, or wood could be used, like 2x4s but that is a little more work. An extra layer of wood on the floor can be added on top if needed also, if that even becomes a problem.
Get rid of the folding step, it will break or bend the camper main frame. A step stool on the ground will have to do and he might not be that agile at 400 lbs either, he does not sound healthy.
I don't think the weight is an issue for the camper otherwise, just his size and agility. If the shoe fits, wear it...
