I too have an old Trillium and spent a good portion of last year resolving leak issues. This included scraping out the old sealant from around the windows
and replacing it with Proflex and installing a new gasket around our door (which was a challange because the frame around the door is warped and required significant refibreglassing).
I also removed the old vinyl strip from the belly band (and replaced it with JR Products 11291 3/4" Narrow Black Vinyl Insert--available on Amazon). When I had the strip off, I replaced the rusted screws with stainless steel ones and used ProFlex to seal the screws, as well as the seam both above and below the belly band.
No matter what I tried though, I still had a stubborn leak that I thought came from the front window area or the belly band area underneath it. I eventually found a spot where the sealant around the ceiling fan vent didn't completely cover one of the screws. I could only see it from a certain angle with the vent raised. Water would pool around that one screw, seep down and sneakily travel between the ceiling and the inside liner and make its way past the window down to the belly of the Trillium. Once I resealed that one little spot, that leak issue was resolved.
The one area that still caused problems was the front window, but only when driving in the rain. I don't have a window cover, and when on the highways, the water and wind combination would push the rain up between the panes of glass and into the Trillium. As a temporary fix, I picked up a sheet of corregated plastic and cut a couple of panels that I would slide between the window and the screen (easy to do with the window open). When travelling, the rain that managed to seep between the window panes would get trapped by the plastic. This temporary fix has now proved so effective, that I'm no longer thinking of building a proper cover.
I'd send pictures, but I live in the Kootenays and my Trillim (nice and snug under a trailer cover) is currently under three feet of snow.