Could be something used your battery compartment for their dinner table with bird on the menu. Wish I could help, but I don't have any idea why your batteries would have discharged like that. You may want to consider installing knife disconnects on them as many of us have done. On an RV, there is something always wanting to suck up battery juice, even when everything is turned off and in store.
Also, if you solve the mystery, please post your results.
2015 Thor Vegas 24.1 in Ocala