Motor homes and boats have a lot more surface area than a car roof. The 1kWh/day is based on the two actual cars with a solar roof, which have a ~200W capacity. Even a paper optimistic about solar cars estimates that doubling that to 2kWh/day needs 800W of solar panels installed on every available surface of a car, i.e. also on the hood and doors. [1]

[1] https://www.researchgate.net/publication/349656294_Future_Op...