Solar panel mounts on domed roof, or like you, roof vents,
An hinged mount on the side of the vehicle locks flush when in motion, 'A' frames out when parked to make panels face 'Up'.
Think glass or plywood rack on the side of the vehicle.
The higher up you can mount with hinge at top will put the panels further up away from road debris.
Parked east/west with panels facing south, you will get a decent amount of sun, and the fill side of the van provides a reasonable amount of square footage.
Parked with panels facing east in morning, you will have to turn vehicle around at noon so panels face west in the evening.
I built something like this for an enclosed car trailer, to add extra panels when parked.
While the prop up can be as simple as a prop rod (broom handle!) I eventually installed a powered jack screw (like the big old satellite dishes had) so it could be powered up, and anytime the ignition started, it folded itself up.
Any welding shop can knock the frame out for a very reasonable amount, all square corners & common materials.
One tip: make the frame OUTSIDE the panels, not under.
Since the panels will be wind drag, panels inside the frame instead of panels stacked on the frame, much less wind drag.
Just an idea, if you can use it, welcome to it.