You know that the 18 amps you're talking about is at 12v rate.....and the cord to the chargers is 110v right? Less than 1.8 amps - and that's at full draw. 12 ga is buttload of conductor.
It's more than enough for the purpose expressed - put it that way. Will give plenty of room for future uses though.
Picked up a 12 gauge 50' at Lowes. Usually 69.00, web had it listed for 41.00