With it being a dually you are doing pretty well with the mileage. If you don't know the age/mileage of the IP and injectors, that is where to start. You can play with the timing some to see if you can get a little more. Adjust your driving habit. Basically, take off slower, drop it into neutral and cost when possible, keep your hwy cruising speeds lower. I did a test with my NA 7.3 e250. Reducing my hwy cruising speed by 5mph gave me a couple more mpg. Remember these trucks were built and geared when the speed limit was 55-65mph.

You could upsize your tires a little and go with some LRR ones, but you need to watch the tire weight. If the bigger tires weigh too much more you can lose mileage due to the rotational mass. Make sure your tires are inflated correctly. Inflating then to their max rating may help reduce rotational drag.

If you have a tach, watch your rpms and look at your trucks powerband. Try and find the most efficient rpm range.

In theory, bolting on a turbo could help your mileage but many times it has the opposite effect since you tend to use the extra power.

Maybe switching all your fluids to synthetics, including wheel bearings grease.

If you do a lot of hwy cruising, changing your rear gear could gain a little bit of mileage.

Not sure if it's worth the $$ for the increase, but that is your call.