I used to have a 73 Trillium
in that lovely Pea Green, and loved it.
I had the same problem.
I removed ALL the caulking on the top of the belly band with a plastic V shaped
putty knife, steel wooled the entire belly band, and resealed with New Clear Caulk.
I also went around the trailer and removed and replaced any rusty or loose screws
with new 1 size larger Stainless steel screws, (actually, one by one, I replaced them all).
I then went and bought new rubber/ plastic molding/ belly band cover (that's hanging in your picture) as it is really cheap 10 cents a foot.
The entire process only took a day, and eliminated the leaking problem, and looked great!
I hope that helps?