The handbrake of the Camper Cabin is designed so that you can turn the driver's seat without hitting it. This is the reason why it sinks back down when activated. This can give the impression that it is not tightened or not working.
So when you try to deactivate it, it may seem a little unusual. To release the handbrake, press the button and pull the lever backwards until it engages. This can take quite a lot of force - perhaps the passenger standing behind the driver's seat can help.
If you have parked on a downhill slope, remember to put a foot on the brake so that the car does not roll.