Please be more specific about the model of coach. Tiffin made an Allegro Bay that was primarily a gas chassis coach but it was also available on the Freightliner FRED (front diesel) chassis for a couple years. It was sold as either an Allegro or Allegro Bay and in lengths from 32-38 ft .
If that front diesel version is what you are asking about, the mpg would typically be about 10, but a few people with a very light foot have managed 12.
If it's not that front diesel model, then it would be a Phaeton or Allegro Bus rather than a "Bay".
2004 American Tradition; 2013 Buick Verano
Homebase in the Ocala Nat'l Forest near Ocala, FL