Armchair quarterbacking. I think the idea of these are an upsell accessory. Integrated brackets on the corner caps might look funny if someone opts out of the guard altogether.Bending vs. mitred would reduced the labor by 50%
Be stronger if the mounts were at the corners. Might add flanges to corner covers that allow the pipe to bolt on. This would reduce the number of holes in the box.... i.e. will hitting the pie 1,2,3,4 times cause the screw holes to leak.
Might also consider making the brackets from 1/8" and/or a 45-90 bend so they crumple vs. damaging the box.
What if the brackets clamped to the tube allowing adjustability on install. Also the tube could run long so one part # fits all. The tube would be cut to length at the back and be finished with a press in plastic cap plug. The tube brackets would be situated over the top of the corner caps on the camper. Installation would be drilling and tapping the corner caps and bolting the brackets on with stainless machine screws and sealant.