Power down your truck and give it a good half hour for all your "standby auxiliary" features to go inactive. Test every individual fuse. If I remember the YouTube video I was watching when I was learning every reason I hated the used Silverado I bought. The guy said on star will eat like .02 volts or something like that. It's a tiny trickle, that a day or two you won't notice, but a spare vehicle that sits a week or two? You might want to look into removing the on star bs or putting in a manual kill switch on your battery cable to disconnect your battery when the truck is off. That's what I ended up doing after spending several hundreds of dollars trying to stop the slow battery drain