15 mpg is pretty damn good for that. I have a 97 Gmc sierra 2 wheel drive 4.3L and that gets 22 on factory rims, and 19 with my rims on (22 inch rims) so 15 is pretty damn good with a big beefy motor like a 383 in it.
and .21 I hear ya about Rcing being addicting.. Its the by far most addicting thing (Besides cigs) I've come across. I know alot of my "truck money" has now changed into my "Rc money"... And the ladies are being neglected, But Fuel gets bad, women will always be there..lol