The way it works with federal SSDI in the USA, is you have to be proven disabled by law. This means your RFC indicates your limitations exclude you from any kind of work. The interpretation of your RFC is also age dependent, meaning the older you are the less likely you will be able to work with your limitations. You can be bedbound and still have an RFC that indicates you can work at age 30, but at age 50 you are disabled. It is also not about $0 income, it is about not making SGA each month which is $1310 for this year. I simplified things in my explanation.
I was able to get a simple form from my county health office which allowed my doctor to declare me disabled by filling it out and signing it. While it is not deemed persuasive evidence by SSDI, it should satisfy the state and county of your disability. Benefits of obtaining such a form include: exemption from jury duty, access to disabled transit services, food aid, property tax exemptions, and etc.