The heavy trucking industry will spend thousands of dollars on fairings and TPMS to improve fuel economy as even a 1% gain will result in many dollars in savings.
None of the fleets use tuners for their engines. The only difference is that the fleet managers decide based on facts and not advertising claims.
Want a guaranteed 15% increase in fuel economy. Drive at 55 MPH instead of 65MPH. Or even at 65 MPH instead of 70 MPH.
RVer's as a whole are the least conscious group of people in terms of doing anything to maximize fuel economy of any group I see motoring down the highway.