Although the windows of my Compact Jr weren't leaking when I got it (it wasn't raining right then), it was clear they had leaked before and badly. The wood frames of the two side windows were rotted and needed replacing.
When I pulled off the streetside window wood frame -- it was so rotted I didn't need to undo but a few of the screws -- I discovered that the screws across the top just barely had a bit of fiberglass to screw into. The problem was that when the trailer was built the cutouts for the windows were a little oversized (makes it easier to put in the windows) but the installer, instead of centering the window in the opening, let the window rest on the bottom of the cutout. Thus the screws along the top are right at the edge of the opening (some missed the fiberglass altogether).
The problem is that there is no room for the butyl putty to work its magic, because there isn't enough overlap between the aluminum frame at the top of the window and the fiberglass above the opening.
So I am repositioning the windows UP, centered in the opening, which gives me enough overlap of the aluminum window frame and the fiberglass to lay down a good butyl seal.
BTW, most people are using "butyl putty tape" which is butyl putty already shaped in a convenient form, typically 3/4" wide by 1/8" thick.
"Butyl tape" is different in that it is not putty at all but instead is double-sided tape made of butyl rubber. It has the advantage that it is a good flexible adhesive, but it cannot fill in crevices, dimples, and other irregularities like the putty can.
One source for butyl putty tape is http://www.rvwholesalers.com/catalog/butyl...e-13-0880x.html
Now that you know what you are looking for you can find many other suppliers, perhaps even a glass shop locally. The discounted internet pricing may look attractive, but the tape is heavy and shipping adds up quickly -- the final price may not be much different than your local glass shop's price.