They offer free checks of your battery and alternator because they want to sell you those items...lol (Pretty sure they still do that.) If both are good, you have a parasitic drain. Do you leave your truck parked for a long time between driving it, such as a week or two at a time? Even a minor parasitic drain can drain your battery. If you drive it regularly, you should do a parasitic draw test. All you need is a multimeter and a little patience. I linked a video below. Those are often caused by someone "hacking" into the vehicle's wiring, such as upgrading the car stereo, or adding an alarm system. (Also a reason I always recommend doing the stereo upgrade yourself by purchasing a kit from Crutchfield...they generally provide a harness that plugs into the vehicle's wiring harness so there's not cutting and splicing of wires.)