Get someone with a lot heavy driving experience to ride with you a few miles, see how you drive. Minor changes in driving habits can make big changes in economy. Sometimes its as simple as lightening your right foot, looking ahead at traffic and lights to lessen use of the left foot.
Getting good mileage out of a heavy high profile vehicle takes a different approach.
edit.
another thought. while the many worthwhile mods proposed could likely improve power or fuel economy, do they justify the cost? That is the reason for your post isn't it, about cost /mile? If so, then mods to engine breathing & fuel system would be more benefit costly than fuel savings?