That's a good question. I haven't spent much time looking into it on here yet. I was more focused on finding the what to look for things. I'll have to keep browsing and get some ideas.
When you have time during the week, contact GM's Heritage Center and ask them decode your VIN. They'll send you a list of the RPO codes and what they mean (the sheet in your glove box). You can also try to tack down the service history of the truck by first checking out the reported information on the CarFax. I was able to contact the dealerships that serviced my truck and they emailed copies of the service records, some asked for proof of ownership, some did not.
Try to determine if the 100K mile services were done. If not, change all fluids and the plugs and wires. Is she 2WD or AWD?
Get yourself a Tech2 so you can diagnose and work on your truck. Get a subscription to AllDataDIY and or buy the shop manual in pdf form from SeriousCTBuyer on ebay. I like having both. Any shop manual works hand in hand with the Tech2 to help diagnose, test and fix things.
Consider a BlackBear tune. This makes the truck more driveable, improves how the transmission operates and can free up some horsepower. My chief reason for having mine tuned by BlackBear was to disable AFM until such time I could physically remove the DOD components and put in a slightly larger camshaft. The tune is your biggest bang for the buck.
If you have little things that are broken, like the cigar lighter covers and worn buttons on the climate control, those things can be purchased on ebay or through Amazon. I put a dash mat on all of my vehicles, these truck's dashboards are prone to cracking and a pain to repair and hurt the wallet. Many of us have replaced the interior door handles, the peeling chrome can cut little fingers. If you like the navigation system, you can update it and consider the benefits of Onstar.
Go to GMCs website and download all the manuals for your truck and make time to read them. If you have the Navigation Radio and the manual is not on GMs website, send me an email, I have it. The 2013s have some cool features that the earlier models do not have like Hill Start Assist, Powertrain Grade Braking Normal Mode and Cruise Grade Braking Normal Mode.
Check the RPO sheet for K5L, this is the Enhanced Cooling Package with larger radiator, external transmission cooler and an engine oil cooler. Look for RPO G80 (Positraction/Locker) and GU6 for 3.42 gears which would give you around an 8,000 pound tow rating. The owner's manual will be more specific based on what you have.
Post up some photos of the inside when you can. We're here to help.
