Our Ambassador had a similar leak. The seal that covers the seam between the front cap and side wall was the problem. If I remember right it was somewhere below the drivers window. I peeled back the rubber trim that fits in the metal track, replaced some screws and caulked. I knew there was a problem when some of the screws were very rusty.
2020 Ford F350 Platinum, 4X4, CCLB, SRW, diesel, 12,400 GVWR
2020 Jayco Pinnacle 32RLTS, Reese Goose Box