2003ChevySilverado

I have a 2003 Silverado and after it sits for more than 1-2 days, the battery does not have enough charge to start it. I replaced the battery back in May of this year and I believe I replaced the alternator a few years ago. The battery I just replaced, was only 2 years old and now the new battery seems to be having trouble. Could this be another bad battery, a bad alternator or another issue? This only started since I have been working from home and not driving the truck daily. The truck only had 136K miles, but I also understand it is 17 years old.