Saturn949,
Next time you are out and about in your 2005 MB Cruiser, stop at a truck stop and pay the small fee to have it weighed.
I'll bet it weighs very close to the GAWR for the 2005 Sprinter model it was built from.
There's your answer.
It weighs a lot, and it pushes a lot of air, so you get poorer fuel mileage the more you push it.
A Scan Gauge II, set to read the % engine Load, the RPM, Gallons per Hour, and your actual MPH (your speed is probably overstated by ~ 3 mph in that Sprinter) will tell you why you are using so much fuel.
You could get it tuned (Bully Dog/GDE/ etc) but that may get your better fuel mileage but cause other problems.
If you learn to manually downshift your Sprinter based RV to keep the % engine LOAD as low as possible, and begin to drive more by the % LOAD and GPH I think you will see 20-22 mpg at slightly higher rates of travel.
If you just put it in cruise control, and let it run, it will use way more fuel because the transmission does not auto downshift when it's needed for best performance or best fuel mileage.
It will lug along, in 5th gear overdrive, using 2x to 3x or greater fuel flow, with the engine at 100% load and losing the horsepower battle (slowing down due to not enough power in top gear).
A quick tap of the shift lever to the left, and voila....you are in 4th gear (sometimes even 3rd gear is needed) and the engine is running up in the most efficient and powerful RPM range (2700-3150 RPM) and the fueling rate goes down by => 50%.
Roger