I too own a big, evil, gas-sucking monstrosity (Ford Expedition) but I use it primarily for towing our camper and occasional family trips. Having 3 boys means the extra room (ie; the kids don't have to be 'touching' each other) is sometimes worth the tradeoff in mpg's. Lol!
The daily drivers for the wife and I are considerably more 'frugal' though.
As much as I love our Expedition the though of driving that every day (and filling that gas tank) makes me queasy.
Getting close to 17mpg in mixed city/hwy is actually pretty good for something that size.
Keep working on your timing and on developing a 'feather-foot' and I don't think 18-19 is out of the question.
A little tip; coasting cannot be over estimated. It works miracles for mpgs so utilize it as much as possible. Work on extending out your coasting distances, which should be easy since you seem to have a semi-set route.
As far as the highway; leave earlier and drive slower if possible. That 85mph run you made was horrifying!
My best advice there is to drive slower (duh) and maybe find a big truck (semi, delivery van, etc) to tuck behind and follow on highway trips. Even without resorting to 'drafting', following a large truck can easily be worth 1-2mpg's.