If this was my trailer and I only wanted to use only VHB tape. I would take a strip of steel strip like pallet straping and clamp it in the channel then used a hand held grinder or a course file and sand down the ridges until your flush with the steel strip. Toss the steel strip after you shave down the ridges. The channel was origionally there to keep the putty sealant from squeezing out all over the place. Leaving a little channel left would be a good thing.
This will make the channel lower than the VHB tape thickness. Now prep and attach.
I would not travel with an awning hanging only held in place with VHB tape. What will happen 5 to 10 years down the road when the tape gets old?
I personally drilled the rail and through bolted where the rail bolts are hidden inside interior cabinets. Where fasteners fall
in open areas I used pop rivets. I used Butyl tape between the shell and the c channel. There is a good write up on this site for installing a bag awning on a 13 foot trailer. I mostly followed those instructions.
I too have an 8 foot awning on a shorter C channel. Don't trim it. It works just fine with over hanging ends.
I dont travel or store my awning hanging on the trailer I keep it inside the trailer between actual use. It stays clean and after 3 years of use still looks like new.