Two sets of power coming into bathroom lights.

Yes, it should be fine to grab power for your fan from the light fixture. Hopefully, there is adequate 12v distribution so the bath light circuit is independent from other 12v draws. Best practice would have the lights on one branch circuit and other devices on a separate branch.

When you find four wires into a location, you can be 99% sure that it is part of a string. Direction of wire exits/entrances has little meaning in the RV world.