You didn't say what cam you have, but it sounds like you might not get much more from a cam change. Something in the 204/214 (or even milder) range would probably be a good choice.
I think the simplest (but not cheapest) way to increase your fuel mileage might be to use one of the simple TBI fuel injection systems like the FAST EZ-EFI product. It will cost you about 2k if you also buy the high pressure fuel pump (which it needs), but the computer control will do a lot to help with your gas mileage.
Fuel injection may not win out over carburetors for horsepower, but it is a lot more predictable for fuel mileage.