Running 65 on the highway not pulling I get about 19.5 mpg average. In town
10 - 12  mpg , not pulling.

I'll guess about 14 -15 mpg with 8,500 lbs on the highway if you dont push
the speed too much. In town, mileage isnt going to be anything to brag
about.

I don't tow a trailer of the same weight, but I can give you the figures I
have.  My truck is a 2004 Chevy 2500HD Crewcab with the Duramax diesel and
the Allison tranny (3.73 rear axle).  I have made no significant engine
modifications with the exception of adding on an auxillary Racor 2 micron
fuel filter.

**City with no load (daily driving) - 18.5mpg, this number is consistantly
predictable every time I fill up.
**Highway with no load (70mph) - 21mpg, but mileage drops when over 75mph.
I suspect that if I could measure it on a longer trip with greater periods
of constant 70mph it would increase over the 21mpg figure.
**Highway with 3000# (camper and gear, 70mph) - 17.5mpg, and I won't pull my
camper any faster.

Recently been to Savannah, GA and back from central Florida so I had a good
opportunity to measure my mileage.

Cheers - Jonathan