Something's not right here.
I tow a 8,300# GVW TT with a Chevy 1500hd, 6.0l, 4 sp 3.73 axle. I kept a milage log on my 3,400 mile round trip to SoCal from BC last fall and avg 9.5 MPG. The MPG between fill up were mostly above 9 MPG towing and close to 13 MPG solo. There was one leg which was exceptionally low at 7.7 MPG but that was running up Siskiyou Summit on a long stretch of 6% grade with engine reving over 4k. Another low one was at a construction zone on I5 where we had to idle for more than an hour.
The GM 6.0l is known to be a gas hog. I tow between 60-65 MPH in tow haul mode when possible and at 55 MPH in Calif which is the speed limit for TT. Somehow a newer version of of the same engine with a 4.10 axle, I believe it is better for towing, only gets 6 MPG towing puzzle me.
