Miles on 7.3L Diesel is not an issue till you get over something like 500K.
That's not true for ANYTHING unless you know exactly how well it was cared for through its whole life. Does it have a K&N or some other "high flow" filter? that'll reduce the life of the turbo, at the very least.... A chip and a filter are common mods that often screw with the durability of the engine. It always amazes me when a chip mfg. offers some sort of "no compromise" power/MPG tune over the factory settings, all by itself. Sure...
For MPG, I'd start by focusing on the basic details, like you should for any car - rolling resistance (air in your tires = 80psi?), aerodynamics (i.e. bed cover), air/fuel/spark (i.e., condition of the fuel injectors, glow plug, air filter, TB), and fluids in the reciprocating assemblies (i.e., engine/trans/diff oils). All that basic stuff should be immediately ran through for a vehicle of that age/use.
Other than keeping it out of the boost and lightening your foot (of course), diesels hate short drives. Until that huge heavy block is warmed up (long time) they get terrible MPG. If you have an 120v outlet near where you park it... maybe retrofit an engine block heater?
A water/methanol injection kit is another option, it should help for long heavy tows, but from what I've heard, be careful with the purity and mix of the water/methanol. And of course it adds expense and complication.