I'd love to see some pictures of the roof. Do you walk on it? Solar panels? Plywood seems like it would be heavy....? I like the diamond plate idea......Maybe glue foam to the roof and then glue diamond plate to the foam.I'm planning a RV with mine and think I will pull the walls to verify the insulation is OK before adding anything else. I've got the beds, racks, heater and A/C out so far. It is going very slowly.
You can totally walk on it. I often use the top of the truck as mobile scaffolding, recently to paint my house with a brush & roller. I have (3) 100W flexible solar panels up there screwed into the plywood with battens, but for scaffolding I throw a blanket over them & then another piece of plywood. It's not super heavy, it's only (2) sheets of 1/2". I wouldn't trust any glue at 65mph. Mechanical connection to the front & side rain gutters is the way to go.
Ya I wish I had started the truck by re-doing the insulation & inner sheathing. I pretty much finished the inside THEN realized this needed to be done.
In the first pic (with me standing on the cab) you can see the orange c-clamps in the rain gutters. A heavy wire goes thru the c-clamp & up to a hole drilled thru the 2x2x8 cross beams. Then plywood screwed into that long-ways, connecting them. Awnings are 8' ARB.