Well, if you're in California, you might just get some medals and whatnot for running biodiesel. Although if it is SoCal, you might have trouble finding a place that has any fry grease at all
A peterbilt 375 gets 4 MPG. Your box truck should not get 5 MPG. As the previous posters have said, it is likely your engine's fault.
Make sure it is running as well as you can possibly make it. Then, for the transmission, find a 5 speed, then put that on your 7.3. If you're feeling even more adventurous, go with a 9 speed non-synchromesh tranny like in semi trucks
you'd have more gear ratios to chose from.
And, yes, aerodynamics play a big role if you do highway deliveries. City will be mostly the massive weight if the vehicle.