0*F here these days. I keep the breaker(s) on, since I've setup the electronics are setup to NOT charge when the temp is below 40*F or so. That said, I have heater running in the camper to keep it above freezing so that when the sun is shining it can charge both the camper and the truck.
Most lithium batteries have a BMS that will preclude charging below ~35F/2C. On some of these you can tweak the temperatures to be higher or lower if you prefer.
By “alternator over working” do you mean charging too much? With no house battery attached, I don’t think that’s what you mean. Or are you thinking of some kind of Alternator run-away with a sudden load reduction? Your truck battery is still attached, so, it isn’t as if there would be NO load. Or am I not getting your question at all?
I think your alternator will be fine. I am presently running my 2015 Silverado with a direct connection to the camper. I have seen 50A going into the camper, but that's a reasonable load for a 220A alternator.