... statement "nothing gained" Tom. Your batteries may be ok but leaving them unattended does have an impact. Plates sulfate, capacity diminishes. Ever perform a real load test on your batteries ( the kind where you place a load on the batteries for 20 or more hours and measure capacity)? That would tell the story. I am not arguing for or against the economics of the cost of electricity vs battery life but make no mistake you are doing the batteries no good letting them sit that long without charging.
David Smead has written a couple of great books on the subject of boat electrical systems and I found them interesting reading... he explains this nicely.
By the way, Our batteries are 5 year old agm's that are always kept on a chartering the winter, this spring while the boat has been out of the water, I left it unplugged for about 6 weeks. Even after occasional use of the batteries to provide light while I worked on the sail drive and did my engine maintenance I had battery voltage of 13.1 volts (and my Fluke is in perfect agreement with the battery monitor in the boat). I finally plugged the boat in this past weekend "just because...". Next week when we get back to our slip I will load test the bank and I fully expect I will have more than 90% capacity of a new bank. I will report back one way or the other....
Bruce