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I would guess that the mpg would be in the 8 to 9 range depending if the engine is broke in yet. In our 40' GS coach with the CAT C7 350hp we get 7 to 8 depending on the terrain and road conditions, but our engine is not really broke in yet since we just have a little over 8k on it.
Our previous '02 36' GS coach we were getting almost 10mpg when we traded it in and it had almost 20k on it. Of course it was not as heavy but it had the CAT 3126 330hp engine.