Don't listen to the diesel bashers. The 6.2/6.5 diesels were great motors. Built for economy not for power. I have owned one 6.2l and two 6.5l trucks. All three had 4:10 gearing, for pulling power. The last pick up, I had for many years. Pulled a big camp trailer all over with it. Empty and a mix of highway and city, always 19.8-9mpg. Pulling the 8000 pound fifth wheel, 15.6mpg.
Key to reliance is like everything, maintenance. I switched to full synthetic oils and gained mpg and reliability. A big help with the injectors, is adding about 1 ounce of 2 stroke oil per gallon of fuel. Helps to lubricate the injectors and gives a mpg gain of 1-3 mpg. Cleaning the "Cooling Stack" (radiator, oil cooler, trans cooler and condenser) is key to keeping everything cool. Cleaning the ground straps once a year.
The only real issues that need to be addressed in the factory set up, is the Pump Mounted Driver (PMD), a set of resistors that were mounted to the injection pump to keep cool, but didn't. Replace and move to the openings in the front bumper.
The oil cooler lines. The GM "snap fit" lines give out and will drain the oil pretty quickly, killing the engine.
The factory waste gate control is vacuum and the lines get brittle and break. I replaced the factory system with a mechanical system and never looked back.
I had other things I did to mine due to pulling a trailer. Gauges (Boost, EGT and trans temp). 5" exhaust and 4" air intake. Water injection to cool the air charge going into the engine. I added a programmed chip into the ECM to go from stock 120hp to 415hp at the fly wheel.
I hated to get rid of it but two kids and three dogs in an extended cab pick up just didn't work. Sold the pick up and fifth wheel and got a Suburban and bumper pull camp trailer. Best mileage I got in the Burb was 18mpg empty and most of the time I saw between 12 and 14. Where I live, gas is pretty bad. Most I have seen in Oregon is 8mpg. Sold the Burb and picked up a 2004 Crew Cab Long Bed Dually with the 6.6l Duramax. But I now pull a 15,000 pound boat. A 33' camper on the water.