You’re going to need those fans with giant, black heat sinks on your roof. To the point that I wonder if the panels heat the car faster than fans can remove it.
Still, you see a lot of campers with solar panels on the roof because a few kwh/day of extra power really does add up over time.
You see a solar panel on our VW camper because a 100W panel can easily recharge the battery that runs the 20W of LED lighting, and charge the occasional laptop, not run a 80,000W traction motor. My parents have a 35 foot fifth wheel with A/C, microwave, and other goodies. It has an option for solar panels. They didn’t buy that option because solar panels ain’t gonna do squat for that load. They figured it might buy an extra half day of boondocking before the house batteries run out. No, you see a lot of campers with solar because they either have a small load like us, or the salesman convinced them it would be a good idea on their 40 foot Class A tour bus.
A 120mm computer at full speed is ~6w and can move 75 cubic feet of air per minute. So, 20w worth of fans can quickly exchange a lot of air.
Anyway, 100W of panels is minimal if your using 20+% panels. You can get that from a 1.5 foot x 3 foot section, a camper can have 20 of those on the roof assuming your starting with mostly clear space.
Car companies are not adding panels because the benefit is not huge, they add complexity and cost, and panels negatively impact styling. Still, you see a lot of campers with solar panels on the roof because a few kwh/day of extra power really does add up over time.