Thanks for your thoughts Brian.
On the other hand, maybe some flexing of the channel supports while driving over bumps would be good? I think I will do a test mount then a test drive before I "nail it down".
The roof does look pretty "saggy" from the top, but there are no signs of my Ceiling sagging from the inside of the camper. And even though the roof has leaked and left some water stains on the ceiling, I saw no signs of dry-rot or even mildew when I examined the first layer of plywood just under the top layer of aluminum.
Actually, once the roof is ever walked on, I don't see how it could be anything other than saggy. (See my recent post Roof vent offset strips/shims necessary? - which I didn't explain very well)
Plus I will be coating with the Liquid Roof EPDM, which the manufacturer claims "can take standing water 365 days a year . . ."
Fabricating trusses is beyond my skill set - would love to see how you do that!
Sam