There are 2 reasons why I can think of to disconnect the battery:
1) When in storage, or not being used, to keep the battery charged up and disconnected from all loads. It could go dead within a few days otherwise.
2) If the trailer is connected to a power source for a long period of time, several weeks or months. The reason here is that having a battery ALWAYS on a converter could litteraly boil the battery away since the voltage applied to it is too high. Some battery chargers are fairly sophisticated, but some are not, and converters aren't good chargers so it really depends on your RV. If you plan on keeping the trailer plugged into a source full time and you are not possitive if the batteries are connected to a converter or a poor charger or a good charger, I would disconnect the batteries once charged, and then connect them up periodically to refresh the charge or when needed due to a power failure. There's no need to have the battery connected otherwise if you're plugged into shore power. The only drawback here is if you loose shore power, you don't have seamless switch over to battery power without flpping your cut off switch.
Explanations can be found here: The 12volt Side of Life (Part 1)
(Some really good 12V battery information here)