Tubs are actually pretty common but so small as to be impractical for anyone over 10yrs old (to use it as a tub and not a shower). You aren't going to find an adult size tub in any production RV's particularly in smaller units.
Assuming you are willing to do a little remodeling and give up space for other uses, I'm sure you could do some remodeling and put in a tub. As long as you are on a full hookup site, water usage wouldn't be big deal.
An off the wall idea: They have inflatable hot tubs. You could set it up outside. If he needs it for cleaning (and not just a hot soak), you could put it under the awning and put up side curtains.