Storing it in a garage in below freezing temp could be the culprit. Regarding removing it after each trip, that's unnecessary unless your concerned about security. If you don't have a battery disconnect switch do the next best thing, disconnect the negative cable from the battery & place it in a plastic or paper cup so it does not have any contact with the battery. My last trailer had no disconnect switch & I left the battery like that for almost 6 months & it still had power although in need of charging. Also make sure it has enough (distilled) water.