I think I have that roof.
I installed a 100 watt panel. I put it so that two of the mounts were screwed directly into the perimeter frame member. There was no doubt about the #10 screws having enough "bite" in those locations. The other 2 screws, on the opposite side of the panel, clearly did not connect to a frame member. There was only a little bit of "bite" as they connected into the plywood. I am depending on butyl tape between the mounting brackets and the rubber roof for grip at that point. Two good, strong mounts, with screws firmly in the framing, and two somewhat weak screws reinforced by butyl tape. I suppose the only other thing you could do would be to move the panel forward or back enough to connect one of those two "other" screws to a framing member.
Mine has been holding up well for almost a year of travel.
-Speak
It's just Mrs. SpeakEasy and me now (empty-nesters). But we can choose from among 7 grandchildren to drag along with us!
2014 F-150 Super Crew Short Bed 3.5L Ecoboost
2014 Flagstaff Micro Lite 23LB