6.0 L fuel economy

I currently drive a factory installed propane powered Dodge 250 Ram van for work. I get about 10-12 MPG average. Where I live, propane costs about 40% less than gasoline, and this is why I bought it. I am looking into buying a high mile, but very well maintained 2003 Express Van Ext 3500 (15 passenger), with the 6.0 L Vortech motor, and dual air and dual heat. I am curious as to what kind of fuel economy those of you who have the big motor are experiencing under daily driving conditions. Most GM sites don't even list an EPA rating for this motor, and EPA seems to misrepresent fuel mileage anyhow. The economy it gets may affect my decision to buy it or not.

I drive mostly rural paved 2 lane highways rated at 50 mph, with a little city driving mixed in. I drive very economically, and never exceed 60 mph (I am very fuel concious).