The tag axle brakes work in harmony with the other brakes on the motorhome for normal service braking. Non of the wheel brakes work as a parking brake. There is a drum brake on the transmission drive shaft that is the parking brake. Even though the tax axle brakes are drum brakes and the rest are discs, they still all work together to stop the coach. There is a thread on here somewhere that shows how the tag works and has pictures of all the parts taken apart. I'll see if I can find it if Dave78Chief doesn't show it first.
Edit: I found it. Click here:
Hydraulic Tag Axle Brake Information