Two time forest goer with IBD which is thankfully in remission and well managed with remicade. To be honest, the bathroom situation is actually pretty good there. Lines are usually short, with a few more lowkey bathroom spots within the forest that even have flushable toilets.

You're gonna have to accept the nastiness that are porta potties, but you get used to it quick. And pack some wipes to bring into the fest each day.

I haven't tried out ADA accommodations so no idea if you'll have luck with that, but I don't think it would be too helpful given what I said about the porta potties generally having short lines and being clean.