Without seeing photos of them, I would say to check out the following three sites for parts, with Vintage probably being the closest to "one stop" shopping:
Edited to add: Hmm, for some reason the forum software is re-naming my links, and the last one is turning "Pelland Enterprises" into their page header of "RV Parts and Motorhome Parts Online Outlet" (when the company name is Pelland Enterprises).
I will re-type them differently; just add the www. and .com on either end and you will have the name and the link.
If you could post photos of specific seals or problem areas, I would be happy to comment more specifically. There are some seals "deep" into the windows
that I have not replaced, but it seems that the usual ones to deteriorate (header seal, seals between the flaps) are reasonably easily replaceable.
You have a good point in that you don't want to spend dozens of hours on them if they are too far gone. On the other hand new ones aren't cheap, and you would almost certainly have to fiberglass in your corners (new ones typically have radiused corners; your old ones are probably square-cornered); so there is some good reason for trying to repair what you have, if they are not completely ruined.