My current coach is the second one that I have had with the Chevy 8.1L Vortec V8 gasoline engine since the Fall of 2006. I have heard in RV forums many times over those years that gasoline engines lose performance significantly as altitude increases. This was mainly from DP owners touting the performance of turbo-charged diesels at altitude. Living in sea-level Florida for the past 37 years, I had no experience to say otherwise.
I have just returned from a 6,700 mile "out west" trip and can now report how the 8.1L engine does at altitude from experience. We run our 35-foot coach right at the 22,000 lb rating for the chassis and we tow a car, so the engine gets a good workout, as gas-powered coaches go.
7.50 = My average mpg for the entire trip.
7.59 = My average mpg for non-Mid-West legs below 6,000 feet.
7.12 = My average mpg for legs running 6,000-8,000 feet altitude.
I was pleasantly surprised that the mpg drop at high altitude was only about 6%. (This included some pretty severe grades at the higher altitudes and a lot of slow tourist traffic in Yellowstone.) Perhaps the engine performance drop at altitude is partially offset by the reduced drag of the thinner air.
We were also pleased with the coach's ability to readily climb all of the big western grades that we encountered and in the Allison transmission grade brake's ability to keep us in control on the descents.
One thing that did surprise me --
7.38 = My average mpg for Mid-West legs below 5,000 feet.
When I came across the Mid-West (eastern Colorado, Kansas, Missouri, Illinois) I took a distinct 3% performance drop relative to the southern tier and the western states. I was puzzled about this until it hit me -- ethanol. If there was any place where 10% ethanol was actually
used in the gas, it would be these corn-belt states. The other areas either had no ethanol statement on the gas pumps or the sticker said "MAY contain up to 10% ethanol."
As a side note: For all practical purposes, the mpg that I am getting with my boxy 22,000 lb Class-A is the same as I was getting with my previous 17,800 lb Super-C. I attribute that to the 6-speed Allison transmission in the current coach vs. the 5-speed Allison in the previous coach.