Does the van have the third seat or the enclosed wet bath next to the side entry door? I was picturing the latter, since you said "standing up in the bathroom." If so the options I see are: (1) share the big bed, (2) mattress on the aisle floor, or (3) build a small cot that goes across the front two seats. None seems very practical for an adult on the larger side.
Thinking outside the "box"...
Tent or enclosed cot/hammock under the awning?
Small trailer (cargo trailer, teardrop, camping trailer)?
We tow a 13' Scamp used by our two teens. We used to camp all together in the Scamp- it sleeps four as long as two are small. But now that they're older and the Roadtrek has come into our lives, having separate spaces is really nice. Of course a trailer makes the whole rig much less nimble.
Scamp and Van Carlsbad.jpg
A lot depends on how often this extra person is going to travel with you.