Welcome to the forums.
First, go to the GM truck dealer, and buy a park brake pressure switch. Then crawl underneath and find the park brake apparatus. When you do, look for the switch you have in your hand, when you find it, and unplug the wires from it, you will probably see a bunch of fluid in the connector. Unscrew the old switch, and screw the new switch in, and plug it in. refill with hydraulic oil, crawl out from under your motor home, and start the engine, and put it in gear. If everything else is still working, the brake will release, and all you will need to do is replace the parking brake shoes, also from the dealer.
I have replaced many of them, one at a WBCCI rally when one failed coming in the gate. IIRC, the switch retailed for about $50.