My 99 K3500 came with dual batteries. It had a few wiring problems (loose terminals, blown trailer brake controller, trailer plug full of mud) all of which I have fixed. After fixing those issues it began draining the batteries so I disconnected the aux battery and under-hood light and the problem stopped. I thought I had a door lock relay stuck & causing the problem, which disconnecting the batteries allowed to open. I have been running off the primary battery for several months with no issues so last weekend I reconnected the aux battery as I occasionally do a lot of starting without driving (pulling a trailer around a field to load hay etc on - start, move 20' then turn it off repeatedly for hours). Sure enough, I went to drive it the next day and the batteries were dead. I did reconnect the under-hood light when I put the aux battery back in the loop, but verified it goes out as the hood closes. I took the aux battery out of the loop and again no problems. I had my local shop test the batteries a few months ago and they didn't report any problems. Any ideas what might be causing this?