Back of napkin, it looks like you get ~3*15 meters of roof which seems to translate to 10 kW of output under good conditions and then you need something like 150 kW to rumble up the highway.
Plus you could use the sides of the trailer for some power too.
Finally even if 10kw can't power the truck. It could extend its range by some amount. Plus that's still a significant amount of power you don't need to buy. 2-3 rooftops worth?
I guess electric trucks would still put a lot more effort into avoiding air resistance because of the lower energy per unit mass of batteries (the aerodynamic improvements can be pretty directly traded for either range or cargo capacity).
I think present day solar cells are a combination of too expensive and too heavy to bother with. That could change in the future.
