Here's my $.02. Your daily driving route looks like all city to me. Getting 14.5 mpg out of a 4x4 dually in the city is pretty good in my mind. To give you a comparison, my regular cab non-OD 2wd truck gets 18-19 on summer fuel. Granted I tow hay wagons almost everyday and I almost never spend more than 30 min at 55-60 mph. The best tank I have ever gotten was 20.8 mpg.
I hate to say it, but you may be at the peak of efficiency