I live in the cold too, (it was -55F this morning, and right now its up to a scorching -50F) and for almost 50 years! 24 V is not new to me. I am opposed to removing batteries, unless for replacemant or cleaning, & especially not in the cold. Or using two 12V chargers for a 24V system... too much delicate wireing to fuss with, in the cold. Since you have a 24V charger (which is great), on the old style slave fitting on your truck, just take a 3/8" bolt, cut off the threads, and stick it in the + side of the slave. Hook the charger + to that, and ground, to something else, far enough away, that there is no danger of the two leads coming in contact.
If you have a new style slave, wire it so you can do the above. You don't have to even raise the hood, to charge/boost the batteries, but most importantly, you don't have to fuss with the concepts of 12 versus 24V.
As far as that block heater plug falling out/your truck not starting, you just need to do your homework. A lot of the newer vehicles today, will start at much colder temperatures then is good for them. Then you wonder why you have lower & lower oil pressure. Things do, and can run dry in there, till you get going. Rule for SubArctic North: Never start vehicle untill there has been enough heat on it, so that oil will drip off the dip stick, when you pull it out.