The are all kinds of parasitic drains that you never think of. They can be as small as the vehicle clock, the various memory sets, the CO monitor, all kinds of itty bitty draws that together add up over time.
The simple solution is to install a battery switch & turn it off between trips. This is something boaters who keep their boats away from shore on moorings are familiar with. The boat is left electronically dead, except for it's bilge pump which is wired directly to the battery. RVs don't need to worry about sinking.
A battery bank in good shape will sit multiple months disconnected & untended.