That sounds like a good option. 20lbs of chicken sounds like a lot! That is over a pound per person if there are 17 people.
Can you do it in a crock pot?
I don't know where y'all are from, but cole slaw or potato salad would be a good side. Slaw is good because a lot of people put slaw on their sandwiches. If you do slaw and not potato salad, you could do a starchy side like corn on the cob, baked beans, or even potato chips. We would eat potato chips on vacation.
The thought of corn made me think of another fun large family vacation meal- a low country boil! That is a little more prep than chicken, though.
ETA:
I just looked and saw you were in WA. I bet no one puts slaw on their sandwiches there. LOL.