You should also make a complaint online with ada they provide a list of attorneys etc. to contact. I have also filed a complaint against a little ceasar in San Antonio to ada and an ethics complaint against a local goodwill. I suggest if they deny service anywhere remain calm find info regarding exemptions on your city website. If its during the weekday and before 5pm contact your city 311 equivalent number. If its after 5 and on the weekend contact the non emergency local police. Try to keep a record of time and individual refusing service etc
Same think happened to me at a San Antonio location. I told them I had a medical condition and I pulled out the individuals with disabilities exemption sheet from the city website and started reading it to them. After a few sentences, they told me I was ok and served me. I'm sure every city including the state should have information on their website with same information. I called the city 311 hotline, and told them I was being denied service based on my disability . They told me that if I have a medical condition that prevents me from wearing mask and I am denied service to call the non emergency police number.
You should get a large group of people to go without a mask and overwhelm them.