Small solar panel for roof vent??????

Assuming you have an appropriately sized solar panel, I see no great difficulty with doing what you propose. You can hook a motor up to a solar panel directly and it spins when the light hits the panel (assuming they're appropriately matched to each other). It is conceivable that an ill-specified and overly large panel could overpower and overheat a motor.

Whether this would be sensible or not depends on what you're trying to accomplish. If the goal is to extract hot air from the RV on summer days, the direct connection has a nice sort of self-regulating and self-contained aspect that has some appeal.