The reason your getting horrible mileage is due to the larger tires. The final drive ratio you now have is drastically different than that of stock. In effect it's almost like you're towing a moderate load at all times now due to the lowered ratio.

If it were me I would re-gear the truck to at least a 4:30 ratio. It will save your transmission and drive train and it will make the largest difference in felt power.

If this is an all stock 6.0PSD I wouldn't start modding for big power without doing head studs or you're asking for a world of trouble. If you start putting a chip on the thing and adding intakes and other performance items you MUST at minimum have a set of gauges to monitor EGT's as well as accept the fact that you're going to eventually blow the head gasket.

The 6.0 is a great motor, but you can't performance mod the thing, run those great big tires on the stock ratio, and expect it to last forever. If you insist on modding, first go with a 4" exhaust. Lower egt's = happier motor = longer life.