You have a drive line parking brake. It has a drum with shoes in it, on the drive shaft. Most are right behind the transmission.
The brakes are controlled by an air chamber with a huge spring in it. The air chamber gets air from the dash switch.
When you reliese the brake, you send air to the chamber. That air pressure, squeezes the spring and releases the brake shoes inside the drum.
When you apply the brake, the air is let out and the spring forces the shoes against the drum, holding the drive train from turning.
Over time, the shoes wear down, far enough, that the spring can't push them hard enough against the inside of the drum.
The fix is to adjust the linkage, or an adjuster between the shoes, so that they sit closer to the drum. That way the spring can do its job.
A good, heavy equipment or truck brake shop should be able to adjust that brake.
You should see both of these under the MH. First is the brake drum and then the spring chamber.