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You are doing well for city driving. I know some cars will give about double the mileage for highway driving compared to city driving. Of course, there is a wide range of conditions in city driving.
I would suggest first that you try letting your engine breath hot air from around the engine. This might mean disconnecting an air intake hose and stuffing a rag into it so it doesn't bring cool air into the engine compartment.
If your ignition timing is adjustable you could try advancing it a little. Don't overdo it or you could get knocking.
I've had good results when retarding the camshaft a few degrees. The torque goes down but MPG goes up. The distributor has to be readjusted to avoid retarding the ignition timing too. If your engine already has variable valve timing this tip might not help.
To get a really big improvement you could try something like the old Crower Mileage System (CMS). The plan is to raise the compression ratio to about 14:1 (by installing different pistons) and install a camshaft that closes the intake valves about 40 degrees later than normal. The result is a reduction in pumping losses and an increase in expansion ratio. Crower doesn't sell this camshaft anymore but I noticed that some companies such as Isky Racing Cams have camshafts for turbocharged engines which have a longer duration for the intake valve. Isky calls theirs "Turbocycle". You can also order a custom made camshaft. There could be a problem of complying with emission regulations if you do engine modifications. Of course this kind of thing gets expensive.
Your wheels look big and heavy. Maybe you can get some wheels from the junkyard which aren't so wide. If you keep the same diameter tires then the speedometer will still be accurate. Alternatively, you could look for narrower tires that fit on your original rims. A less aggressive tread should reduce rolling resistance and wind resistance.
I doubt you would gain much from very high tire pressures and it would give you a rough ride and extra stress on the suspension. I would also be concerned that a tire might explode while you are filling it if it is getting weak. I think the main problems are that your engine is large and you have a heavy vehicle. The large engine will consume extra fuel at light loads and the high weight will require extra horsepower when accellerating and climbing hills.
I suggest that you keep a bicycle in the truck and use it for short trips when you don't need to carry much. Keep it hidden under a blanket so it doesn't get stolen.
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