Everyone, take a look at the OP's post about having around $10,000.00 to spend. Their definately not going to be able to get into a MB Sprinter or a late model class c, so those of you who are posting about 15 mpg with a Sprinter are not giving the OP any useful info.
At that price their looking at a 15-20 year old class c, and will more than likely have a 460 Ford or 454 Chevy V-8, with the old 4 speed auto with od. Those combinations were famous for lousey gas milage, even brand new. Lot of power, but drank gas like a wine-o.
Now that engine is 15-20 years old. You think it's going to get better???
Just flat out, good gas milage is not going to happen, no matter how you modify it with "air tabs", "flow thru this or that", or whatever "snake oil" someone tells you will work. There is no "magic" transformation or additions that will significantly increase gas milage.
Grumpy