There are 3 seals to complete window restoration. The pic below shows the section detail. 1) glazing tape is butyle that holds the glass in place to the back of the frame 2) glazing bead is a gray vinyl extrusion that is the finish seal to the front of the glass. Both of these items are available from Airstream Dreams. 3)The 3rd seal is no longer available as a semi-t-slot bulb seal and has been replaced with Hehr part# 03 0009, a adhesive backed sealed foam that inserts into the channel of the window frame.
The glazing bead is pressed into place, I used my thumb. You should also compress the material as you go, ie, force the material towards the side you started at so as to compress an extra inch or so along the way. When it gets cold outside and the glazing bead contracts, the end joints won't seperate. When I installed the new foam seal (#3) I left the backing on and inserted the material into the channel. An ice cream stick will help with this. Its possible that the foam can just be stuck onto the top of the channel, but I inserted mine and it seals OK. It didn't come with any instructions, so your choice.
The 2nd & 3rd pics below shows an old piece of glazing bead dehydrated after 40 years & the new glazing bead. Original bulb seal, some replacement I found that's a tad too small and the replacement foam.
To remove the windows from the trailer to do all this, mine had a set screw on the top middle of the frame, under the drip shield. The drip shield had to be removed to get to the set screw! Once the set screw is removed, lift the window and slide it out. There is a ton of technique getting the old parts off and installing the new. Basically hold your tongue in the right position, go slow, don't use anything sharp and be careful! I managed to only break one piece of glass! I also replaced the front and rear windows with Lexon, which is a whole other story.