My much smaller OFF-OAD POPUP trailer doesn't have any needs for 12VDC when in OFF state so everything is disconnected when the BATTERY SWITCH is operated.
Some of the larger trailer may have an electric tongue jack, or leveling jacks, or maybe an electric door steps etc that are all operated by a POSITIVE SWITCH ON action. These will stay connected to the BATTERY TERMINAL for you to use when needed... As long as a positive switch is required to use the 12VDC coming from the battery then you don't have to worry about your battery draining down.
Anything always connected to the battery when left ON will drain down your battery in a few days or so if it is turned ON... A lot of trailers may have the SMOKE or GAS detectors wired directly to the battery. These will also drain down you battery over a short time period...