The camper “costs” about 10% in mpg to pull. I don’t believe the cross section of a solar panel would produce a significantly additional amount of drag. (IMHO as an engineer.)
If you’re considering making it removable, I wouldn’t let drag/efficiency be a factor. Rather consider, do you want to alternate between going off-grid and kayaking or cycling? Or would you rather have the convenience of mounting it, and forgetting about it? (That’s how I’m leaning, but I have a rack on my car.)
Another consideration is that removable solutions run a small additional risk of improper installation each time you re-install the panel.
Mine is potable. Left mounted it could be more susceptible to damage from falling branches and just sun aging. I don’t use mine on all trips. If the trailer is in the shade I can move the panel to the sun. I have a connection at the transom like my shorepower. The plug is keyed so it can’t be connected incorrectly .
CLC’s prototype has leading and trailing edge fairings to reduce the drag. The other thing to keep in mind is the angle of attack. If the panel slopes toward the back it’ll be generating lift, as well as drag, and will try really hard to fly off the roof. Sloping forward it’ll be getting pushed down onto the racks.
For absolute best performance, you’d want to completely do without the racks while driving. They disturb the clean airflow over the top of the camper much more than the panel. You’d also want the panel centered or have 2 panels spaced symmetrically on each side of the overhead fan if you are keeping the rack. For that matter, the tongue box can also mess up the airflow unless it’s in the wind shadow of the tow vehicle.
As a practical matter, though, it doesn’t make that much difference, especially if you drive at reasonable speeds, so just do what’s convenient for you.