Congrats on the new trailer. We have had our 260 for about 2.5 yrs and still love it. The battery will drain fairly quickly from the stereo, CO detector, etc as you have guessed. I installed a very easy disconnect, which is available at most auto stores. It just replaces the negative battery connection and has a thumb wheel that turns and opens the connection, thus disconnecting the battery. Takes about 2 minutes to install and has worked well.
The other really great mod is to mount a PVC fencepost under the frame to store the sewer hose and connections. If you are interested, search the forum here and there are several write-ups about it. Hands down the best mod I have done as it gets the hose out of the bumper (I still keep a reserve in there) and all the fittings out of the storage compartment. Cheers.
2011 Surveyor SP260
2012 Ford Expedition