I've had this happen twice now. When trying to level the coach with the engine running apparently the HWH system commands the DC compressor to run even though it's already at pressure and causes it to cycle the pressure relief valve until I pull the power or hit the emergency stop on the HWH panel. After getting the coach level I re-established power to the compressor and it stayed off. I bled the HWH tank until the compressor started and it pressured up normally and shut off. I'm stumped.
Do you have an "air tank" switch by the driver's elbow and is it OFF (it should be)?
If OFF, it could be a failed check valve in the aux compressor plumbing to the air tank switch solenoid line preventing the 2 systems from being isolated or the air tank switch solenoid or valve itself weak or failed or has debris on the valve seat/face.