parking brake does not engage at all

We have a 1991 Sunclipper with an Allison AT 542 transmission. Recently the parking brake will not engage. We have to block the tires to keep it from rolling whenever we stop.
When the linkage is removed from the brake on drive shaft and it is manually applied it works fine.
What could be the problem? How do I fix this?
Thank you, Joe and Marg
parking brake does not engage at all

Joe, not sure about your MH, but on mine the adjustment is on the parking brake handle itself. It's adjusted by turning the end/handle of the brake lever clockwise to tighten, counterclockwise to loosen. It adjusts the cable going to the brake itself. Hope this helps.