For any seams on the roof, I would highly recommend ParrBond to lay a bead along the seam, then cover that with some EternaBond AlumiBond tape. You won't have to worry about those seams for a LONG time.
ParrBond @ Vintage Trailer Supply: Parbond
EternaBond Alumibond: ETERNABOND Roof Repair Tape, 4 Inx 50 Ft, 20 mil AS-4-50R - G2917153 at Zoro
The ParrBond CAN be found in a caulk-gun size if you need it.
The EternaBond seemed pricey to me, but it works WONDERFULLY! Well worth it!