Usually there is a wire but it may take some digging to find it. I used to install Fantastic Vents and found the older Safari model did not have any wire going to the front or rear vents. We often had to run wiring to the nearest light. CAUTION: if you are getting a Fantastic Vent with rain sensor and you hook it into your light, make sure you bypass the light switch by the door or on the wall. If you do not bypass, when you shut off the light, the fan will also go off and the rain sensor will not be active. If the vent is open and it rains it will not close. In short, the Fantastic Vent Rain Sensor models need to have a full time hot going to it.
In the newer Safari models, Airstream realized owners might want to have a powered vent and actually ran wires to the 'area.' Sometimes you have to (from on top of the trailer) search between the rib and inside skin with your hand to locate the wire(s) often taped to the inside of the outer skin. Occasionally, only a hot wire was run and you had to use the rib as a ground.
It's not a difficult job but if you're not familiar with 12 volt
electricity then its better to get someone who is familiar with doing this type of job.
On most of the 70's, 80's and early 90's Airstream trailers, wiring was bundled and run down each side of the roof vents. One side carried 120 volt wiring and the other 12 volt
wiring. For replacing non powered vents with Fantastic Vents, we would tap into a ground and a full time live wire to power the vent.