As suggested, the bladder can be inflated while in Drive, not necessarily with the parking brake off. Many a bladder has been damaged by a copilot or late entrant reshutting the door after the driver put the coach in gear with or without his foot on the brake.
The following diagram may help, as there is commonly an adjustable solenoid that may need tweaking or cleaning. A hose or wire there could be the culprit as well. My first easy and obvious test would be checking the bladder integrity by leaving the door open, start the engine, and with your foot on the brake or parking brake on, put the rig in Drive to see if the bag inflates. Sounds like you already know the result. Then check the solenoid/regulator function and all connections to and from. Don't overpressuize the adjustment, as per the 5psi warning. It may be as simple as a tech or rock bumped the adjuster knob to completely off.
Joel