Unlike your service brakes, that take air pressure to apply the brakes, the parking brake chambers are spring loaded such that it takes air pressure to release the braking action on the rear wheels. Therefore, a loss of air pressure will ensure the parking brake is applied. If you search "air brakes" you will find plenty of info with diagrams explaining how the entire system operates. This general info should understood by all drivers.
Certainly chocking the wheels will not hurt and is prudent if the site is not level.
Lou
05 Travel Supreme Envoy