Cross-posting from the "Do You Tow An Airstream" thread...
We did about 5000 miles going from Chicago to the LCDC and then through New Mexico and into Big Bend NP over the last few weeks. I now have a Trail Tailor front bumper with winch and the Lance 2185 is definitely 500-600# heavier than the old Surveyor 235RS.
I got anywhere from 6.5 to 7.5 mpg on my trip out west and back, typically going 70mph. My wife managed as high as 9.6 - not sure if she was going mostly downhill in OK or if she benefited by following an 18 wheeler for a couple hours. Interesting to note that on some stretches like from Carlsbad and Guadalupe Mountains to Big Bend I felt like maintaining 60mph was a chore and barely managed 6.5 mpg, continually downshifting to 3rd, whereas I was doing 75 in Illinois yesterday afternoon without any effort to stay in 4th gear and got closer to 7.5mpg. (Edit: ok more like 7 mpg @ 75 and 7.5 mpg @ 70).
I watched my transmission temp for much of the trip too. It seemed to sit around 201F most of the drive - about 5F warmer than last year. Not sure if that was due to the heavier trailer than before or to the new steel bumper and winch affecting the air flow. I did see the pan temp get up near 210F regularly when dropping into 3rd on long uphill highway passes, and as high as about 235F (with the TC pushing 250F) on one very long mountain pass in CO on my way to the LCDC. I may look into mounting a secondary transmission cooler in the spring (also doing a full transmission fluid flush again along with the diffs and transfer case since by spring I'll be at 90k).
I also weighed my rig on the way home yesterday afternoon. Wow.
Steer axle: 3460
Drive axle: 4620
Trailer axles: 5200
Measured GVW: 8080 (3460 + 4620)
Est GVW fully loaded towing: 8350 to 8400 (8080 + 200 + ~70 to 120)
Measured Gross: 13,280 (3460 + 4620 + 5200)
Est Gross: 13,550 to 13,600
Edit: My trailer TW is about 800#, so I'm pretty close tothe trailer GVWR of 6000# and without the trailer my drive axle is probably closer to 4000-4200# when otherwise loaded with kids and gear
That was with a 45-50 gallons of gray and black water, which sits over or behind the axle. I did not have a chance to weigh empty or unhitched - we'd been in the car 2.5 days at that point and I was already pushing 7pm to get home. My wife and middle kid weren't in the truck, so add 200# to the above, plus a mostly empty cooler and an empty rotopax and I can see the total weight of my rig approaching 8400#.
The good news is that I'm under the GCWR of 14,400. The bad news is that I'm well over the GVWR of 7,385 by 1000# when towing. While I have medium-duty rear springs (Tough Dog 240# coils), my fronts are still the 500# stock rating. The next time I do a suspension upgrade I will move to heavier fronts (675#) and rears (at least 340#, possibly more) - the truck towed and braked fine, but going over dips in the highway with the trailer in tow it was quite floaty - more like a Caddy than I'd like.