The letter says there's an "urgent need to act on disability inclusion" and urges major studios to appoint permanent disability officers.
The number of disabled characters on screen "continues to severely under-represent the actual US population living with disabilities", it says.
"Due to years of misrepresentation in the media, social barriers, and chronic ableism, the deaf, hard of hearing, neurodiverse and disabled communities continue to be underrepresented and disrespected in the entertainment industry."
Around one in four adults in the US have some form of disability, according to government statistics.
A recent report found that 3.5% of TV series regular characters were disabled in 2020. A separate study put the number of disabled characters at 12% in 2018 - but found that most of the portrayals were negative.
A previous letter in 2019 said that, among the 61 Oscar nominees and 27 winners who had played characters with a disability, only two had been authentically portrayed by an actor with a disability.