I think of the sidewalls as much more supportive than the roof. I think I'd want to span the roof and use both gutters even if you intend to load and unload your stock from the side.
Most commercial awnings attach to the roof curvature in order to get a rain seal.
I was the lucky recipient of a free Thule awning and made an L-bracket adapter out of 3x4 aluminum stock in order to attach it to the side of my existing walk-on roof rack. Works well enough in all but the worst rain but I'll have to sew up a short top skirt in order to help screen the awning against bugs.
We had previously used a home sewn $50 knock-down awning that used four emt pipe sections and a surplus tent ground sheet. It was lightweight and strong enough for its intended purpose to bridge between the van and a quickshelter. Worked well to give us a big screened porch.
The new Thule awning gets more use as a quick sunshade but the quickshelter is much roomier.