I'll do my best here.
The ScanGauge provides readouts from the truck's OBDII system. (https://www.scangauge.com/products/scangaugeii/ for reference). I use it to keep tabs on IAT and coolant temps, as well as a "guessimate" of mpg. (All of my mileage figures are hand calculated, from the same station, same pump, unloaded every time. I fill up at the beginning of every trip, and log fuel purchases and mileage until the next time I can fill up at my favorite station.)
The 6600# weight was from the scale at my local gravel pit. ~30 gals of fuel, driver + passenger, my toolbox/chains/straps/etc onboard, but not my 50 gal aux tank. My scale ticket from Alberta on 4/1/14 says 1960 k (4321#) front axle, and 1350 k (2976#) rear axle. Truck, with me, my gear, and about 50 gal. of fuel onboard, no trailer. That is a certified Canadian scale, so it's probably the most accurate. Total weight of 7297 pounds.
Tires are Firestone TransForce A/T, 265/75R16, on stock steel rims. About 50% tread, run at 65 PSI.
I run bumper pull travel trailers or horse trailers. Usually no 5th wheels right now, no in-bed campers at all. Curb weight and sizes will vary, from 2000# up to 10,000#, 20' to 37'
I just bought this truck a couple months ago, haven't had time to get it completely outfitted yet. I've got a set of gauges, boost/pyro/temp. The only one installed right now is a trans temp gauge, mounted in the pan. The others should be going in this week, as I've got some time off. I will also pull a diff cover (probably the front) to verify the gear ratio of 3.54.
I'm also going to try and make a couple of pulls on the dyno at a local automotive college. I want to make a pull with stock ECM programming, as well as a couple with the Smarty on different settings.
I don't have much experience with these trucks, having mainly run bigger, IH and Cat motors in my past, where fuel efficiency wasn't nearly as much of a concern. I know there are aerodynamics issues with this big of a truck, things that I will be addressing. My only other comparison is word of mouth from other drivers. Just met another yesterday, with an identical rig, except 2wd. He claimed to get 14 loaded, 25 empty, but didn't have any numbers to back it up. YMMV, as they say