Hmm... I've never seen that kind of trim, but it sounds like the plastic is pushed into an aluminum strip that has teeth to grip the plastic. Which of course means you'll probably ruin one or both by pulling it out, but it might be the only way. Maybe you should scout around for replacement trim in case it's needed.
'87 Fleetwood Tioga Arrow
If you're in a hurry, you shouldn't be RV'ing