The usual culprit is the LP detector.  Based on what I've read from others, it's usually wired into the 12v system.  On my old popup, the radio's LCD panel was aways on too.  That said, I know I've let it sit disconnected for up to a month before, and it was only then getting into the low range (i.e., below 40%, 11.9v)

My new HTT seems to chew through my group 31 battery much faster than the popup did, and what I plan to do at some point is check the amps that each 12v circuit is pulling to see if I can narrow it down.