Lol, J/K. Nice motorhome, BTW. I am sure that a Banks tuner will work on your rig, as long as they use the same motors as the SD trucks. Past that, anything you can do to increase horsepower but not increase fuel burn will increase mileage, so an exhaust would be a good option, too. Lastly, remove anything that you don't use that weighs you down and make sure your tires are inflated properly and your engine serviced in correct intervals.
No matter what you do legally, you're dealing with an EPA compliant engine burning crappy EPA fuel.
I just recently began thinking of going from our Jayco Eagle to one of their MHs but am really surprised that, from what I've seen so far, which isn't a whole lot, they are not offering a diesel in their Ford chassis. No faith in the 6.7? I'm pretty sure I won't go with the big Ford gasser. No excuse, I just gotta have an oil burner. Haven't priced the Seneca. Ouch.
There's a bit of good news for you though. You can buy a lot of gas with the money saved over a diesel if it had been available in the E450s. Just guessing, you're gas penalty is only the difference between 7.5 and ten or so mpg with the diesel, LESS the half a buck a gallon diesel price differential. As for power, well, take it easy and enjoy that nice rig.