It all depends on the park, the part of the country, etc. I was planning out the route for my upcoming trip to Yellowstone from Louisiana over the weekend, researching potential overnight stops etc. I noted the web site photos for more than one of the potential stops showed a U-Haul and similar trucks parked in the RV spaces, I suspect these sorts of places would have no issue with a converted school bus, yet other RV parks will not even allow nice looking motorhomes over 10 years of age on site, and some of these are not particularly nice looking locations.
In addition to this some publicly operated and many private parks have minimum age requirements, I have seen ages from 21-25 listed.
Personally I think they would be better off buying an older Class C motorhome for this adventure, better chance of getting their money back selling it in the end. There are lots of $5,000 motorhomes out there, some may even have 5,0000 miles of driving left in them.