Yep - its the parasitic drains. Quite a few of them in most trailers.
You can buy an "on-off" battery disconnect such as the Blue Sea 6006m or one that allows you to select battery bank 1 or battery bank 2, or 1+2. If you know you will add a second 12-volt, than buy the Blue Sea 9001e.
This is the 6006m
I will also add that you might
have damaged your current battery if it had been discharged too low and it might not hold a charge anymore.
You can test it various ways yourself or take it to a battery place and they'll test it for you.