For this sort of job I've just applied a length of aluminum tape. This is the high quality tape, about 2 and a half inches wide, used to join sections of ducting in ventilation systems. You could apply two layers to beef it up. I would rather trust this to keep out water than a heap of screws. It's also very quick. For belt and braces, I've put the screws on with sealant, and then used the tape on top. This is a useful trick with the joint between the roof and the awning. When you walk on the roof you might see this joint opening up as you move. This is what caused me to tape this joint.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."