My similar ‘06 Marquis/C13 hasn’t hit 7mpg yet, even on straight level flat ground.
One interesting observation on the road this summer for the first time with the Marquis: Increasing my speed from 60 MPH to 65 MPH on cruise control did not result in any reduction in fuel consumption. I think that this is because the “Sweet Spot” for the C-13 is around 1650 rpm, and 65 mph puts me pretty close to that engine speed.
Just a couple of other observations:
For a 45’ coach weighing north of 50,000 lbs, full of liquids and Doritos, with a C13 Cat to average 10 mpg on relatively level ground, under almost any conditions is almost impossible. This raises the probability that some undiscovered variable was in operation. I have gotten wildly varying figures when I thought I had filled the tank, but failed to notice that the coach was leaning toward the fill spout, for instance.
My Alladin system is consistently inconsistent in its read outs, for another. It is currently showing a 5-7% lower odometer reading (compared to my GPS).
Also as previously mentioned, a strong tailwind can also provide surprising increases in mpg figures.
My previous coach was a 40’ MONACO Windsor with a 370 horse Cummins. It was more than 10,000 lbs lighter than the Marquis. Without a tow car attached, I could achieve a consistent 10mpg.