Larry and Diane,
The H3-45, yes, indeed has a different window set up from the XL
If you look inside at your window sill, you could see few screw covers in the corian sill. Pop off the caps and unscrew the screws and a recessed portion of the corian sill should come out exposing bolts that holds the bottom of the window frame to the coach .
Check to see if the bolts are tight.
The way this works, is the H3-45 side windows of your vintage have a frame. Removing the bottom bolts will cause the window frame to swing out from the bottom. Pushing the bottom out a bit, and looking at the backside of the window frame, you will see a butyl rubber style weatherstripping similar to what you have on your car door. This is how the window frame seals. Dust and grit maybe compromising the seal incase the windowsill frame bolts were loose or the window wasn't pushed in far enough when the frame bolts were tighten. Just a gues.. Caution,tho, if you tip the window frame from the bottom out and up too high, (perpendicular to the coach side) the window frame may fall out of its top track.
Our experience is with our last coach which was a Marathon 2000.
You could possibly need a new window frame seal on the back side of the frame from Prevost.
This is just a guess, tho.
Jim and Chris
2001 Featherlite Vogue XLV 2 slide with Rivets-current coach, 1999 shell
Previous 22 years,
We have owned every kind of Prevost shell but an H3-40