Important to realize that shading (caused by the box, tree, roof vent, etc) of a single cell will cause a consumer panel and any other panel(s) installed in the string of panels to drop proportionally and on some panels, to the level of the shaded cell.
If you plan to on rely upon solar charging, think hard and twice about installing a box on the roof with roof mounted solar panels. I own a Thule box, not for the camper, and it would be a bear to install and remove on a dialy basis to ensure direct, unobstructed sunlight to the solar panels. Further, I don't want to be relocating the truck's postiion (several times per day) to the solar axis to ensure unobstructed sunlight either.