Hi Bruce,
I apologize for my late post... I thought I posted way back, but couldn't find it...
I have an '08 Cruiser FF Xtra - 190XT Toy Hauler (almost 20' long) and I pull it with a '13 Chevy 1/2 ton crew cab Silverado Max Tow, with a cab high camper shell. It has a 6.2L gas and a 6 spd Allison Automatic Trans with trailer tow switch & manual override switch, and 4x4. The truck is factory set setup at a maximum tow weight of 10,900 lbs.
My first pulls with this truck and trailer (partially loaded) and at 60 - 65 mph were much better than my old Blazer at about 7 mpg, and ran at an average close to 10 mpg. However when fully loaded, including a 600# ATV in the trailer, my mileage dropped to an average around 9...
I pulled the wind deflector (Wing) off the Blazer and modified it to fit the Silverado and it made a huge difference. With the wing mounted on the top of the camper shell and positioned as close to the trailer as possible... about 7 feet forward of a direct line from the trailer (a maximum of 6 feet is recommended), my mileage jumped a solid 3 mpg... Two trips to AZ & S. Cal and back to MN (about 10K miles total), my mileage averages now at a solid 12 mpg... AND without the ATV, mileage runs closer to 14 mpg, and NO wind effect when passing or meeting trucks...
In my set up, I always pull in trailer mode (It acts like a converter lock-up) and found the Cruise control, and speed to be two critical variables. With cruise control ON, the truck swifts often and mileage drops... with cruise control OFF, I can drive a little faster downhill and a bit slower uphill to keep tranny in the gear I want, increasing mileage.
Good luck and let us know how you are doing...