Your battery manufacturer may provide a table showing depth of discharge vs voltage for various amounts of load. If not look at page 32 for Lifeline AGM batteries: http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf
The table includes a 120 hr rate which should match minimal electrical use such as a refrigerator control panel and a propane leak detector and maybe an LED light or two. The results do not need to be highly accurate. You are only looking to be over an approximate 50 percent discharge on most occasions. Unless something major is running such as a TV or furnace, I use about 12.2v as a warning and 12.0 or 12.1 as the action level for starting up my generator or killing all the major loads.
