The BIGGEST problem with solar on cars isn't really the cost or drag, it's the WEIGHT. Solar arrays weigh a lot, the more often you drive your car, the more the weight penalizes you. The more solar you add, the more weight you have... and the more power you need to overcome the weight. Depending on the efficiency of the panel and how often you drive the vehicle, you could actually be taking an energy hit rather than gain by having them on your roof. It's also unlikely solar arrays will ever be efficient enough with future technology growth to make them worthwhile to be on a cars roof. The other problem with putting panels on a roof is that cars are not always outside... It's a poor engineering/design tradeoff to put them on roofs, and I would bet someone good money that we'll never see them used on car roofs in our lifetime or ever. Same thing goes for battery powered cargo planes and long haul trucking, the
weight to power ratio for batteries is never going to be good enough for mass adoption compared to combusting fuels.
What is a smarter from an enginering and design perspective - is to power charger stations with solar. This way you can A) guarantee the panels are always out in the sun, and then B) the panels are not adding weight to a car.
Edit: I'll add, one reason you see solar panels on RV's more often is because RV's generally sit more than they're driven, and they're hard to park in shaded spots
