What year is your MH? If it's older, like mine, you may not get much better. I have an older 87 454 and have had it tuned, proper air pressure, don't carry excess items, run about 13K and get 5.5. My mechanic says the only thing left to do would be to rebuild the carb. But for that it's about $300+ and the amount of travel we do and the price of gas it would take a long time to recoup the cost. If you haven't had it tuned up try that first and if not you may not have much hope of increase vs. cost to do that.
2017 Grand Design Reflection 303 RLS
1997 GMC Sierra 3500 Dually