Better fuel mileage from 87 305 TBI?

[chevymotocross]

I have an 87 silverado 2wd swb with the 305 tbi and 700r4 transmission. It also has p265/75/15 tires. True dual 2.25'' exhaust from manifolds back, with high flow cats and dynomax mufflers. There is a new EGR valve installed. I only average 13mpg with a mix of highway/city sometimes pulling an open 5x8 trailer with 2 dirtbikes. If I'm on the highway, I try to keep it under 75. In the city driving, I drive with a light foot. Engine, although has high mileage, is in good tune. Any tips on getting better fuel mileage with the 305 tbi? I was hoping to get an average of 16?