It could be where you are driving. I have a o7 E450 26.5' Lazy Daze. I generally get around 9.8 mpg in the rolling hills of the Mid Atlantic states.
When I visit the Shenandoah's the MPG drop to around 8.5.
I have seen 10 to 11 mpg when on a long stretch of flat road, staying below 60mph, and traffic is light enough that I never have to touch the brakes or gas (think steady state momentum).
For tire pressure, I run 70/80 lbs/sq in front/rear. I generally travel with at least a 25-40 gal (half to 3/4 tank) of fresh water.