Just wanted to share some info I've gathered during our restoration work on my Dad's '78.
A good replacement rubber (filler plug) for the interior trim of the window frame (not the flocked window gasket) can be purchased from Duncan Systems (Part #2-132-014470):
The product was inherited when Duncan Systems purchased Atwood Mobile a few years back.
For the flocked window channel, I purchased from Metro Moulded Parts, Inc. I need two (2) WC-22 96" and one (1) WC-22 48" to fill the gaps. You really need a 108" run for each side, but I couldn't find any custom lengths. I think there are some good options if you just check with your local glass shop.