How long was it sitting since you last plugged it in?
There are phantom loads like the CO/natural gas detector, the control board on the fridge, the radio, etc that drain power every day. You probably drained your battery so low that you damaged it, and an overnight charge likely did not restore more than a surface charge to the battery. Grab a new battery and test, if it works then you've learned a costly, but not very expensive lesson. If you'll be continuing to store the trailer away from power you can protect the battery by adding a cutoff switch, or by adding a solar panel (trickle charger would probably be sufficient, but for not a lot more you can add a 100-150W panel that will also allow you to do some boondocking, and isn't going to walk off like a trickle charger panel might).