The roof space being at a premium, I'm thinking of installing flexible panes mounted on a pivoting frame that would travel locked onto the side of the camper, but when camped could be rotated like wings to follow the sun. The weight shouldn't be extreme, and another advantage would be that it would provide shade for the windows both while traveling and while deployed.
Truth be told, I've never seen another installation like this, so I'm not sure if there's something I'm missing which would make it impractical. However, if it would work, I could reasonable get 1200watts of solar without taking up any roof space at all.