First and foremost, if you have to worry about a difference of 2-3 mpg between this rig or that rig, you need to rethink what your getting, and more importantly, why your getting it. This is a lifestyle, not a hobby, and lousey gas milage is a fact of the rv experience.
Also, when you really think about it your talking about just a few hundred dollars over a trip of 5000 miles. So, if you have to worry about $200 bucks over a month long 5000 mile trip, look into a Prius and stay at a Motel 6.
Each rig will have it's good and bad, it's likers and haters. If you can make do with a smaller 24-25 foot rig, and won't be towing anything heavy, a MB chassis may be ok.
If you want a larger rig with several slideouts, and towing a car, you need a Ford based 450 rig. It has the power and grunt you'll need.
Yes, you will end up spending a few bucks on the suspension, but once done it's a very comfortable rig to drive. Also, with a Ford (or Chevy) based rig, pretty much any Ford/Chevy dealer can fix/repair a breakdown.
MB, try and find a MB dealer that will repair a motorhome breakdown. Not going to happen. Specialized dealers only, unless your real lucky.
Personally, we have a 28 foot Lexi, towing a Subaru Crosstrek. I have a Scan Guage and average 6-8 mpg. Up hills/mountains, less. Downhill with the wind behind me, more.
But, I'm not concerned with the milage, it is what it is, and there are no magic add-ons that will change that. We bought it when gas was $4.00 a gallon, and didn't worry about milage or cost then. Now gas is $2.00 a gallon, and still don't care about milage or cost.
Milage concerns seem to be the very first thing a new owner thinks about, rather than planning trips, and seeing the sights.
Find a rig that suits your needs, and don't base your decision on whether it gets 7.27634 mpg or 9.8736 mpg.
Gas guage settings...
Full.......just crusin along
3/4.......still crusin along
1/2.......think about getting fuel
1/4.......look for fuel