Really bad milage

[Surfdaddy]

Throughout the history if this truck the mileage has always been about 10mpg or better even with a 10ft cab over camper on it. I have all the mileage and trip data from when it was new. I have been noticing what I thought that the mileage was getting a little worse since all I did was drive it around town. Yesterday I drove to the swap meet and had a full tank. I was going between 55-63 mph and I could truly watch the gas gauge go down. I drove about 100 miles on the tank and it was empty, (really empty since I ran out of gas with 1/8 left on the gauge and two blocks from my friend’s house). The question is what would cause such bad mileage. I have a petronics ignition, the timing is right on what the book calls for, new plugs and wires.
Granted it’s a V6 and totally stock with 410 gears and 28 inch tires but I would have thought it would get better than 6-7 mpg. The idle mixture is a little rich and I have the screws turned out about 2 turns from bottom. But according to the book there is no adjustment once you are off the idle circuit.