Disconnect battery when not using?

Yes, as said above you need to disconnect the batteries when you leave the trailer in storage. If not the phantom loads like the refrigerator and propane detector will drain the battery in about a week. Once a battery drains it starts to take damage. The longer it sits drained the more damage it takes.

So yes definitely replace with some new batteries and then disconnect them every time when you store it.

Edit: To reflect that the batteries are disconnected when left in storage.