Removing a locker surround

cyberyacht

I want to remove the locker surround that provides outside access to the bed space. It is a Thetford frame. I've removed the inside trim plus all the screws from the outside but the mastic is holding it firmly to the outside wall of the van. I'm reluctant
to apply too much BF&I as it's made of plastic. Even the judicious application of a chisel driven in between the frame and the body shell isn't helping to prise it off. Is there some cunning dodge to release the grip that the mastic has on it?