Last time I actually followed my exact MPG on a tow was a few months ago.
I averaged 7.429 MPG towing about 7000# on a 500+ mile trip, the next trip I did I got about 7.53 on a 600+ mile tow with a very slightly lighter trailer
Both of these were almost all highway with 1/2 the trip being with an unloaded 2500# trailer
Ill take an educated guess on regular unloaded in town driving at around 9-10 MPG
Its a 2002 GMC 2500HD 8.1L
Ive been looking into a diesel very heavily for about a year and a half now, but cant find the right one, Im very picky haha. I calculated around 150/month in fuel savings (as of gas prices about 3 months ago)