I have basically the same truck as you, and ODS, with the 3.73 diff for mountain driving, and I have a similar terrain as you do, lots of hills and twists on the very coast of Maine. When I drive only around town here, I get 15-16. Your lower mpg is likely due to the lack of oxygen, as said, and maybe some driving habits peculiar to your routine. However you may benefit by making it easier for oxygen to get into and through your motor. Consider this: when I opened up the air intake and exhaust, to flow better, my mpg did absolutely improve noticeably under all circumstances. I pulled that snorkle on the air cleaner box off and put a piece of 1/2" hardware cloth over the hole to keep rodents and leaves out. That small neck on that snorkle is to reduce air noise, as they have done since the sixties; we would cut that neck off of our Impalas back then. Apparently GM marketing people had found that enough customers were made fearful by intake noise when they goosed it. Getting that snorkle off will increase the opening by double or more. And I went to a 2.5" Flowmaster muffler, and the larger 2.5" exhaust pipe that Walker/Dynomax was making, so all was larger from the CAT back. I certifiably gained mpg and gained more snap on low end acceleration. There is expense to the exhaust part of it, but it was worth it to me, and may have paid for itself. Maybe also make sure your CAT is not getting plugged, too.
