Have you just parked the campervan and turned it off but when you open the driver's door the vehicle starts beeping? This is an alert signal. This typically happens if there's something the vehicle wants to remind you of, such as the key being in the ignition or lights being left on.

Please check the light's settings to make sure you have turned 'em off. Take the key from the ignition and open the door to leave the vehicle. The beeping sound should have stopped now.