I did not read all of the replies but the problem I encountered probably was not covered (too dumb to admit). I had a similar problem. When I started checking, I had a substantial drain at battery when parked and "nothing" on. For some reason, I went to a switch or switches that I think are on the driver side windshield post (you can tell I have not been in it a while). They are not marked as to purpose or on/off - I could not find a light on). Changing the setting of the switch removed the drain. The switch was for the heated mirrors and who knows when turned on.
We got the C to attend our grandsons' ballgames. Never did - my wife got sick. We used it while she took out-patient treatments at the world's top center for multiple myeloma (UAMS in Little Rock, AR). We have perhaps used it once or twice since. It sits in the building for months. Hopefully I can get her to agree to sell it sometime. It was too small and if we ever have to go back for treatments, I will get a trailer with a little more room (permanent bed).
The bottom line is that all batteries will die if left long enough. The last one I purchased was a glass mat at about $400 but even the literature for it says to have it hooked to a battery minder if left unused for an extended period. I now have a trickle charger/battery minder on both the chassis and house batteries (I used trailer light connector to make a quick connect for the hard to reach house battery). I do not know how long that has been but over a year - perhaps two. Everything seems to be holding. A week or so ago I disconnected minders to move RV further in building. Both batteries are still holding at max. I need to hook them back up.
Good luck