It will truly depend on your driving style and if you are towing. My previous coach was a 34.2. Fully loaded coach by itself weighed approx 20,800 lbs, at that weight I averaged 7.5 to 8 mpg driving conservatively. Cross a mountain pass and it will drop to under 6 mpg. If you are a lead foot or in a hurry, mileage can drop to approx 5 mpg!!
Once I started pulling a CRV, the mileage dropped to approx 6.5 mpg, again driving conservatively. I invested in a ScanGuage II, plugs into the OBD II port and mounts on dash. I'm able to monitor real time gas mileage and drive accordingly, I really like the ScanGuage, it allows you to see where and when you burn the most fuel while driving. On a long road trip, multiple fill-ups, it really adds up!
I love Scan gauge too. You can set it up to monitor almost anything that has a sensor. Like trans temp, voltage, horse power, intake air temp, current MPH, avg MPG, Tank MPG, Day MPG, current speed, avg speed, ete, etc,
This post is opinion based on experience.
2015 Hurricane 27K
2013 Sea Ray 220SD