You're correct. Only the roof is likely to give significant power. Been there and done that, on the opposite side of the planet though. :)

The "something" on the picture I attach... was built in some squat in Eastern Europe. It had a flat roof of approximately 2 x 1.5 m, all of it solar panel. Solar panels weren't great back then. Typically it charged its 4 KWh battery in a few days of sunshine. Only during midsummer (18-hour days) was there any chance of a full charge in a single day.

Unlike the van, the "something" required a smaller inventory of tools to build. Instead of lawnmower motors, a Chinese electric motorcycle motor was used. Sadly it's now retired due to metal fatigue. :( Lesson: never build a structure that flexes out of aluminum - aluminum has no fatigue limit, any flexing will lead to cracking.