Butyl Tape.
I can't remember the width or thickness. Dh will be home from work soon and I can look in our Rv catalog and tell you. I do remember this,one time we wrapped two rows of butyl tape because it didn,t bring the window out far enough. This is a two person job. I used googone to clean up the gummy residue from the side of the trailer and then used my regular cleaner. Let the area dry before reinserting the window. Some butyl tape will squishout,cut off woth a putty knife. DH favorite tool is an old butter knife. Then apply Lexel caulk to the edge of the window frams where it meets the siding. After some road jarring check inside trim ring screws for tightness. We replaced all 24 window screws with stainless. Any screw that is rusty needs to be replaced with a stainless one. Ordered a huge quanity of screws from MCFeelys.