Need advice on making Reasonable Accommodation Request to stay remote (California)

I am a remote employee in California. Recently, my employer (CA-based company) has issued an RTO mandate for 2026. This initial mandate is only for employees within a commutable distance (which I am not), but I can smell blood in the water. I know they will be coming for us remote workers eventually, and I want to be proactive about securing my WFH status.

I have a history with IBS. I need to be near a bathroom during flair-ups, which would make my potential 2+ hour commute (if RTO goes into effect) extremely difficult. I’ve seen my doctor regarding my IBS symptoms, but not extensively. I have not (as far as I’m aware) received a diagnosis.

My question is — what do I need to do to get proper medical documentation to support my inevitable RA request? Can I simply have my doctor write a letter?

Truly appreciate any advice or insight.