I don't know much about the chassis that particular motorhome is based upon, but a couple of things come to mind which might have skewed your number to the lower side. Though, I'm tempted to say that 12-14 MPG is probably pretty optimistic if this is a gas engine motorhome.
That said, how long was your trip? Did you have to fill the tank several times, or are you basing your calculations on a half-tank of fuel? The more times you "fill the tank" you tend to average out the fact that tanks don't always fill to the same level everytime. If you base it on a single fill, you the pump may have kicked off early when you topped it off to leave, and then didn't kick off early when you refilled.
Also, what terrain were you travelling over? A lot of mountains, or mostly level?
Was your trip recent? If it was during a cold snap (we've had a few in recent weeks), your mpg will be lower in the colder weather.
Can you tell us any more about what chassis the rig is built on? Again, I think 12-14 might be optimistic, but I would think 9-12 could be within reason. So you getting about 8 isn't that far off.
But, rather than dwell on MPG....HOW DID YOU LIKE THE RIG!!! :-) I've seen some Dynamax's and they looked nice!
Whatever the MPG, Good Luck on your new RV!
~Rick
