Just under 8 on long trips that included a lot of east coast grades and pulling a toad. Combined we're 17000 pounds.
I'm beginning to think that the 55-60 I'd been running is no better and maybe not as good as 60-65. I learned years ago with a 1998 V10 towing a fiver that faster meant less downshifts. I'm also using cruise less, unless the road is literally flat. I shoot for "60-65" and if grades take me faster or slower, oh well. There are passing lanes for that.
It's clear the 6.0 Chevy gets a little better gas mileage than the V10. I've also noticed from my experience with the fiver and what I read here: The early V10's rated 275-HP seem to get a little better highway mileage than the year 2000 and later with 305-HP.