Battery not charging

Back in Dec, after a long vacation and my Tahoe being parked for close to 40 days, I returned to a completely drained (and bad) battery. I knew it was going out prior to leaving so I figured I'd just replace it. Picked up a Optima Red Top and life was good until about 2-3 weeks ago. I drove from Delaware down to Central Texas for some schooling for the Air Force and noticed the volt gauge severing fluctuating for maybe 1/2 the drive ... didn't think anything of it since all 01-06 Silverados and Tahoes that I've driven have done that. Started having issues starting a week and 1/2 ago. Turn the key, Tahoe tries to start, turns over and then dies like there's no power. On Thursday I go to start the Tahoe and everything dies. No lights, no dings, no nothing ... take the key out, take a deep breath and try to start the damn thing. It started and I've had intermittent starting problems ever since. Everything I've read says that the alternator is probably the culprit. Since it's dead right now I can't troubleshoot anything ... after class tonight I'll get it jumped and take it to AutoZone for testing. Anyone have thoughts? I've not had any of the normal parasitic draw problems that I've been reading about (lights, AC, etc staying on). Any insight would be incredibly welcome.