2008 Chevy Avalanche my battery keeps running down

My 05 Avalanche has the original battery at approx. 50K miles. While working on the instrument area, I needed to turn on the ignition to move the shift lever out of my way. I left the ignition on for maybe 10 minutes, and that was enough to kill the battery. The battery came back with a battery charger. I was surprised that it took so little time to drain the battery. There is no "accessory" setting on the ignition switch in this vehicle; when did they do away with that? If you try to listen to the radio of leave your parking lights on just a few minutes, it's probably enough to require a jump to get going. A plug-in gauge shows the alternator is working fine. Battery is just at 11 volts before I turn the key in the morning, and then I see 10.5 or 10.3 volts. Looks like 5 to 6 years is the lifespan of this small OEM battery. IMHO, this battery is not enough for a truck with all the electrical items that are standard equipment.