From my experience, dealers have more leeway than that. We ordered the exact same 2018 Serenity as the OP about a month ago and were able to customize a trailer that the dealer had in the pipeline but that hadn't started production. The model and color combo were the only things the dealer couldn't change - we deleting the 2nd A/C and powered awning that were originally ordered and added the awning package and solar.
We pick it up on the 19th and, like the OP, will also be towing it with a Grand Cherokee V8!
Got an update from the dealer today. Looks like I have lucked out, the projected completion date is 8/14/2017... Guess I got one of the dealers reserved slots.. Whoohoo!
This website is not affiliated with or endorsed by the Airstream, Inc. or any of its affiliates. Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.