I added a Banks full package to our Ford Triton V-10, 275 HP version(1999). Let me say first that you need to read the Bank literature and not what some sales person tells you. No matter what you do, or don't do to your RV, the biggest single factor in the mpg that you get is your driving habits. If you drive aggressively or fast you will get poor milage.
The Banks kits will do what they say they will. That means that it will give you a very significant increase in horsepower and in torque. The effect that will have on mileage will depend upon what you do with that added performance. If you continue to drive at the same speed and to take the same amount of time to accelerate, you will see a marked improvement in the mpg from the motorhome. But if you do as most owners do and enjoy the improved acceleration and even begin to drive at higher speeds, that will mean little or no change in mpg. You just can't have it both ways. In my case, I seldom accelerate much more quickly that before the Banks and I never travel the highways any faster than before. We drove at about 60 mph before the Banks was installed and we still do that. In rare cases I will take advantage of the ability to gain speed more quickly, but not as a general practice. Our motorhome was getting about 7 mpg before we put the Banks on and today we get more like 8.5 mpg. But we have good friends who put on on a very similar motorhome to ours and who drive much more aggressively that we do and they rave about the increased power but moan about the lack of any significant change in mpg. It is just a matter of what you do with the performance.