I haven't seen anyone address the fact that modern cars have a lot of parasitic draw on them regardless of tow disconnects. My 2014 Silverado will drain a battery in just over 4 weeks if not driven in that time. Between engine computers, transmission computers, electric steering modules, radio and entertainment center draws and anti theft systems, there is a constant drain taking place. It may be convenient to blame one thing or another, but I'll bet it's the combination of all the drains that is draining the battery.
On my Silverado I installed a latching relay (aka "salesmans switch") on the negative battery cable. The relay uses 12v power only to switch from on to off. This totally disconnects the battery from anything electrical requiring me to reset the electric windows and to select a radio station again each time the power is disconnected. The radio presets are saved, but the particular one I was listening to has to be reselected by pushing it's button again.
I hid my control switch, a simple temp on/off/temp on switch in the grill. So when towing I can hook everything up, leave the key in the switch for the steering wheel to turn, use the fob to lock the truck, and then kill the power at the grill. When unhooking, hit the switch again to restore power, use the fob to unlock and continue as normal switching the transfer case etc.
During the summer when we aren't travelling in the MH, the truck sits for sometimes 3 or 4 months without being used. I've wired a Battery Tender plug up to the grill (again hidden from normal view) so it can be plugged in and kept in top condition while it sits in the carport. I've found over the years that the Battery Tender brand has very few failures and my batteries in my seldom used vehicles last 6 or more years versus the normal 4 or so.
As always, your mileage may vary. I have 23 wet acid batteries that I maintain in my fleet of vehicles. All use a battery maintainer when not in use. Battery Tenders on single battery applications and multistage chargers/inverters on the multi battery applications.
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