Just a few general observations. The chassis builder configures the engine and driveline combination to deliver a specified degree of performance as measured by not only fuel economy but delivered torque and service life. One has to assume that this configuration was selected by the coach builder to be as close to optimum in terms of tradeoff of economy vs performance for their intended product. There would be little benefit to ignore mileage vs power in either direction, so ostensibly whatever the net result is, it's whatever compromise is practically achievable for this price point. In short, it is what it is. So inherently, there's only so much you can change one performance parameter without affecting others. Maybe you could stick a 100mpg carburetor on there, but likely the net result wouldn't be useful. Practically speaking, what would be a considerable change in fuel economy? 5%? 10%? Is that enough to get you to the point you want to be? A 10% MPG increase starting at 9MPG barely gets you to 10MPG. Over many thousands of miles this can add up to appreciable fuel expense but for an RV that might travel 5K a year it's a 50 gallon difference, not even a single fill up. Not saying one wouldn't do a tune up to get that, just saying that at this end of the scale moving the needle a few tenths which is about all you're likely to get, is lost in the noise.
Mark B.
Albuquerque, NM