I have 2 x 85W panels on my van that are wired in parallel and the same SS10 controller. Good combo. The panels have junction boxes. I wired them as you would wire a pair of 12V batteries. There are two wires connecting the + and - terminals. The wires to the controller are from the + of one panel and the - of the other. Three or more panels would suggest a collector junction.
I would not worry about an air dam. The leading edge of a cabover camper generate a lot of turbulence as do the vents and cross bars and AC unit if there is one. The solar panels may be the most aerodynamic things on the roof.
An air dam would only be effective in a horizontal plane. With the panels angled downward, they are pushing through the air, not cutting through the air. Best example is hold your hand outside the window of a moving car. Holding it horizontal, does what? Tilt it? Yep. Now you understand.
The only thing I would worry about is exceeding the panel's designed wind load limit. What is that? Maybe 90-100 MPH? Try telling your son to just keep it below 100 and he will be fine. Larger panels will have less load limit (Wind/Snow) than smaller panels. That is why I shudder when people go for that LARGE panel. But, they probably are never in snow areas.
Air trapped under the panels while stationary? You have it better than most, in that the air will heat up and move through the space upwardly, same as tunnels are designed to do, one end of a Tunnel is lower than the other end so air travels through naturally. The warmer the air, the faster the air will move.