Best MPG for a 2wd 67-72

[72 cheyenne pdx]

I get around 12-14 with mix. Straight hwy, I am around 15-16. I am carryin g a 500lb custom canopy/camper. Stock 350 motor, except the carb (Edlebrock 600cfm), and new 3" single exhaust. I know, at the time I could afford dual exhaust, and I was tired of the exhaust coming into my cab whenever I had the windows down. 3.73 gear, th350. I believe I might get better if I didn't have the camper on it. Well see in the future.