Mark
I believe your coach has a driveshaft mounted drum brake on the back of the transmission which is cable operated by a remote mounted Maxi brake chamber.
The yellow parking brake valve supplies air pressure to the Maxi brake chamber diaphragm to over come a large spring and release the parking brake when the yellow valve is released the air exits the Maxi brake and the spring applies the brake by pulling on the cable .
If you brake won't hold the vehicle there are several things that could be wrong
1) the brake may be worn out or out of adjustment ( not common as the brake is only applied when the vehicle is not moving
However if for some reason the brake has been craving it is possible to ware the brake shoes and cook the brake drum .
2) the cable may be broken or stretched
3) the spring in the Maxi brake chamber may be broken.
Here is link to a Maxi brake so you know what it looks like yours of course will has the cable attached
https://www.powerbrake-drivetrain.com/spring-brake-chamber-midland-pb-ksmr3ph50-r/WARNING
The Maxi brake chamber has bolts holding the outer can with the spring in it on DO NOT REMOVE THOSE BOLTS the spring is compressed and under heavy load and is VERY dangerous if freed it can cause injury or death the Maxi chamber is not serviceable there are no parts available to service it for that reason it must be replaced as a unit.
Hope this helps
Eric