Dealer won't budge: What extras should I ask for?

i won't comment on the price but that mileage is a major issue.

you'll still need a brake controller. the tow/haul button is strictly a feature of the factory tow package and has nothing to do with the trailer's brakes.
BUT all you'll need is a $10 harness to hook up a brake controller. it's easily a 10 minute plug'n'play job. not worth asking it for an extra.

and the Yukon will easily be able to tow the Roo. don't need a HD truck.
i own a 23SS.