Anyone with auto experience can do more then one thinks on a big truck, with more room to work to boot. Any old school hot rodder knows how to adjust valve lash, one thing I learned real quick how to do on mine, takes about 45 minutes to an hour, including inspecting valve springs for breaks and any mechanical malfunctions under the valve cover, can't see paying a shop $300 for this everytime.
A/C repairs are not hard, do some research and go for it. If A/C compressor is getting too loud when on, replace it before it throws a belt and strands you. Be sure and replace the dryer and accumulator as well, junk tends to collect in them. Harbor Freight sells a vacuum device to evac the system for $15 bucks that requires compressed air. I've done it on the road before using the truck's air system. Oh, and most tractor stores, Kubota, John Deere etc can make you're bunk A/C lines usually cheaper then the truck shops can. A/C compressor, dryer, accumulator and freon ran about $375 total. Beats paying a shop north of $1200 to do the job.
Electrical items not hard to diagnose with a test light, always and negative and a positive. No power, move toward the battery until u find it. I was able to diagnose my brake light switch AMU with a test light. AMU's have been discussed here before as a major problem. Install them correctly and they are not a problem.
Injectors, something else I have learned to remove and replace, but be sure and read up on the procedure, important things like replacing o-rings, evacuating the fuel and oil out of the cylinders. I used Justanswers.com the 1st time and was able to gain the Cat repair manual pages online for $18 (your choice what you decide to pay the responder). I have also learned many times injector streaming problems (excessive smoke, rough idle, slight miss, poor mileage) are caused by dirty injector tips, which can be scraped clean. Get one of the bench top jewelers magnifying glass thingys and look for the injector nozzle holes. Mine Cat has 4 holes per injector. Use a razor blade and scrape em clean. I do this on mine as a general mntnce item every 80 to 100,000 miles since my motor is over it's life expectancy (805,000 mi.s vs life exp of 750,000).
My alternator stopped charging one morning, took it off and carried on my bike to the dealer as a core and they even R & R'd the pulley for me at no charge. About three different wrenches or socket sizes and your back in business.
Water pump is a little more complicated. Although i have replaced it in a truck stop parking lot one time, the last time it went out in northern Cali, to get back to TX I trimmed down a piece of soft drift wood I found to fit the weep hole, coated it with black RTV and tapped in. Made it home with minimal leakage, maybe lost a quart in 3,000. Got home and replaced in my drive.
Had a nickle (don't ask) take out my radiator and a chunk of a fan blade while on one good load, replaced it in the street by my son's apartment while on a 2,000 mi. load, saved the load and the labor (did buy my son a tank of gas and sent him a check for $100 for his help). Wasn't easy but we did it in a matter of about 10 hours, including evacuating and recharging the A/C and running 180 mi. round trip to FL of Houston for parts, they even had the correct Cat radiator fan is stock. Load was a weekender so we still made it on time.
I'm researching gaining an ASE cert thru online courses on big rig brakes to do them myself, but until then, even though I'm sure I can do it, I'll leave them to the shop for liability reasons.
I realize some of these are considered repairs, but I'm just throwing some things out there that are not that hard to do if you can turn a wrench. When I started driving some 6 yrs ago, many times I was told not to rely on my hot rod days experience and that the truck is a whole new "kettle of fish", but really it is not, just have to apply yourself a little.
Peace out