Purchased the 25 foot strip from Vintage Trailer Supply and proceeded to get it done. I actually bought enough to do the bottom strip on all the windows, but not the vertical side parts. These are still in decent shape though dry.
I applied a car rubber restorer; "Gummi Pflege" a German product that you basically roll on from a tube with a foam applicator on top. This stuff did a great job on the rubber parts on my vintage Porsche 911 and I found it to help the vertical rubber on the Airstream.
The horizontal bottom strip looked like this: cracked, dry and missing some sections. Its amazing that the windows did not leak...much.
I used a Dremel wire brush to clean the old trim off totally for a good tight fit.
I also purchased some 3M Trim Sealant which is terrific stuff. Find it at any auto parts store. The new rubber came pre-glued but I don't trust that to hold, so I ran a bead of 3M on the window and the back of the rubber. After letting it dry for 5-10 minutes I pressed the cut rubber pieces onto the window frame. The 3M is a little forgiving, but not much so you have to set the trim right or you are likely going to tear it by trying to re-position.
After an hour or so the job was complete. The windows look better and the trim helps keep the water out.
Here is the 3M sealant. Great stuff, but hard to get off your hands!