Wow, surprised at the low MPG reports. I suspect it's mostly due to the speed at which folks drive. We have a 2000 32' Monaco v10 triton and have averaged 10 mpg not towing but did have a 1300 foot "mountain ridge" to climb over. I do not drive fast, mostly 55-60 mph. Towing my saturn vue on a similar trip I got 7.5mpg. I was most impressed with my last trip up over Mount hood where I got 8mpg (not towing). The pass level is approximately 4500 feet and I started at approx. 100 foot level. If I was getting 5.5mpg in a smaller rig I definitely would be concerned about moving up. If you're bent on driving 65-75 then your milage will suck. There's a reason why the government set the speed limit to 55 in the 70's oil crisis. Pulled this off the interment "According to the U.S. Department of Energy (DOE), as a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.21 per gallon for gas (at $3.00 per gallon).
So, the Fuel Economy Benefit of driving slower is between 7 and 23 percent. The equivalent gasoline savings is $0.21 to $0.71 per gallon."