Our 2004 is a Marquis Sapphire, C12 505 hp, rated at 45 feet and weighs about 42,000 pounds full of fuel and water and loaded ready for camping. We have always towed a Chevy Avalanche which weighs another 6,500 pounds. We have rarely seen over 6 mpg and those times were probably because we didn't get the fuel tanks topped off consistently. Over the long term, nine years and about 35,000 miles, I would guess our mileage averages about 5.25 mpg. My brother's 2000 Tourmaline, C12 425 hp, is 40 feet and obviously less weight, plus towing a lighter Honda car, leads to his mileage being much better at about 7 to 8 mpg on a bad day. If memory serves, he has seen over 10 mpg on the flat and level freeways. Plus... my brother generally drives slower than we do, at 60 to 65 mph versus my 65 to 75 mph. When I have driven all day at 60 to 65 mph, I get 15% or so better mileage.
On our annual 5-day treks between Hermiston, OR and Yuma, AZ in October and April, we stop to fill with fuel at Winnemucca, Amargosa Valley, and Quartzsite. Filling in Quartzsite leaves the fuel tank nearly full when parked for the Winter in Yuma. That's about 1,275 miles using about 250 gallons of diesel. We usually stay overnight at the Weatherby Rest Stop north of Ontario, OR, Winnemucca, NV, Hawthorn, NV, Amargosa Valley, NV, and Quartzsite, AZ. At most of these stops, we will be dry camping instead of paying for an RV park. We carry a Honda EU3000is generator in the pickup to run overnights while dry camping unless we need to run the Onan 12.5kw Quiet Diesel for cooling.