Pamela, I too have an 84 Burro. The first thing I did when I got it was to seal the windows, starting with the worst first. On the inside is an aluminum retainer ring with screws every few inches. Going around the rim, loosen the screws a little at a time, continue around until all are loose. By doing a little at a time you don't put too much pressure on the remaining screws and bend the aluminum. If possible have a helper catch the window on the outside. If no helper is available, open the window and hold with your arm so the window doesn't crash to the ground. While the window is out, clean it thoroughly, including the weep holes in the outer bottom which often retain water if dirty and cause the water to enter the trailer. Scrape off the old butyl and silicon. I cleaned mine in my kitchen sink.
Then clean the area around where you removed the windows. This takes time and patience. There are many threads on this site explaining what people used for cleaning off old butyl and silicone. When replacing the windows use butyl rubber tape around the exterior, this gives you a good watertight seal. You can buy the butyl tape at any RV store or online. After you tighten the screws,( again tighten a little at a time, working your way around the window) you will find that the butyl rubber has squeezed out around the window, especially if done on a warm day. Carefully trim the excess butyl without scoring the fiberglass. Donna recommends using a plastic knife. At this point you should not have any leaks
around the window, however, the interior seals may leak during a blowing rain. Even this should be minimal if you have cleared the weep holes.
While you're in the mood you may want to check the overhead vent for leaks
. Use the same butyl and the same proceedure to reseat the vent . Avoid silicone. Good luck and enjoy your new Burro. rb