Fist off - I am not going to do this job myself, I would just like to try to do this without cutting open the ceiling. We have a 110 GFCI outlet in our garage and I would like to add a 30 amp plug that would come from it's own breaker for our Airstream trailer when I'm working on it in the driveway. The distance from breaker box to the garage wall is about 20 feet. What is the possibility of tying on the new wiring to the existing wiring, pulling it back to the breaker using the existing wiring, and then pulling it all back to the garage together? Is this an unreasonable request to ask the electrician?
In order to pull new new wiring with existing wiring, the circuit would have to be in conduit. Assuming you residence is like most, your system is more likely to be Romex-type cable (NM). If this is the situation it can't be done because the NM is stapled to the framing. So if you talk to the contractor you might ask him about the easiest way to achieve your task.
Hope this helps.