Dead batteries in 04 Duramax 3500

Thanks for input. Solar not an option as our unit is shedded when not in use. All things that plug in are never, ever, left plugged in when we're not driving, except my "duh" moment leaving the new back up camera plugged in. GPS is a Garmin windshield mount that is only plugged in accessory outlet when driving. New back up camera is only connected to power from truck when driving, otherwise it's unplugged. We unplug the camper from the truck completely and just use shore power of we want to do anything in it at home. That's why I'm looking, now, at glow plug switch and door switches that control interior cab lights as they have been known to stick on. Was told to pop hood and check fuses one by one to see what is always hot even when functionally turned off to see what is actually drawing power sitting. Checking for maybe intermittent battery cable or other short somewhere as well. What a pain but what else would I do for entertainment waiting for it to be travel time..... but that's another thread entirely!