Might have to dig a little but some of the archives will have floor plans.
Your going to have a couple problems. Biggest being the door. It's simply not wide enough. Somebody with good fabrications skills, Metal brake and possibly a tig welder could skin the door. Cut the frame in half add about 8 inches you want it around 32 inches so the person is not busting the knuckels and elbows when getting in. Not sure if the factory hinge is up to the task.
Next problem is this. Something down one wall is going to have to go. This is going to cause two problems.
1. natural left and right ballance is going to be thrown off. It's important. It's going to have to be delt with.
2. Loss of major storage space.
I would design with a Goucho with a fold up table in the front. that will maxamized floor space. When the table is up and extended it will give the person plenty of room to get at the table. When its folded they can back up to the couch or the folded table to watch TV or chat. The Goucho will form a bed. Some had a slide out lower section to form a double bed and the back was used to fill the extra width. I would think this would be something a person in a chair could handle operating without much problem. It would be something like a Futon couch with some storage drawers under it.
Bath is going to be another fun point. The standard rear bath in some of the units may actually work and have the room they need. The yhave a tub with a low rim. It might work out quite well with a bath chair for them. THe problem is rear bath tend to be curb side bath tub. You cant widen the door to the curb side because of this OR your going to have to remove the tub and the person will have to bath in the campground baths. It would open a lot of floor that way but it's if you want to live without the tub. You can (depending on the bath lay out) widen to the the street side. The side you widen somewhat dictates that the galley would have to be same side as the tub.
I would think it would be easier for the person to navigate a street side galley because of the oriantation to the goucho if the table is up. Possibly there is a model that had a street side tub but none come to mind. www.vintageairstreams.com
has the photo archive and you might want to look at it for some ideas.
Good luck with it. Keep us informed. I would like to see what you come up with.