I'd check with Dometic customer service for sure. Or your RV parts dept. It isn't that unlikely that there would still be replacement parts available, 1991 isn't that old in RV years, after all. Also if the current seal isn't actually ruptured you could try giving it a good cleaning and making sure you can't flex it back to life somehow? Or [b]slide a piece of something inside the seal in the area where it needs help in pushing against the box. I do like Donna's foam idea, though... sounds like it might work if you manage to make good corners seams.
I'm an appliance repairman,
and I normally have to heat up the seal with a hair dryer(and sometimes insert Kleenex)inside an empty area.
Usually works pretty good, if you can no longer get new seals.
Another "trick" I use,
is to remove the entire seal, clean it real good, and soak it in extremely HOT HOT water for appx 1/2 hour, then re-install it, and then use the heat gun/hair dryer the help re-shape it to it's normal position.
You can use a piece of tape to help keep door closed for an Hour, while it's setting up.