I was able to run wires through my ceiling but it wasn't easy, and it may depend on where your closest power is. A lot of RV roofs are trusses with styrofoam in between them. If you can obtain power from a source that is between the same trusses as the fan you are looking to power you might be able to poke a hole through the styrofoam to get to it.
If you take down the vent housing you might even find the vent pre-wired and the wires hidden behind the housing. If they aren't there you will still see the foam blocking around the vent opening. I used a wire snake and with a lot of wiggling it to push it through the foam was able to get it to come out through a light opening a few feet away. I then pulled a wire from there.
Some other RV's are even fiberglass insulated in the roof and that makes it easier.