[JRTJH]

JGriff,

Let's compare "apples to apples":

If you leave the trailer outside (unheated) and plugged into shore power or with the battery cutoff switch "OFF" and a trickle charger connected, then from an "environmental perspective" you've got essentially the same environment, if you store your batteries on your covered porch with a trickle charger connected to them.

I don't see anything to preclude you from storing them on the porch, provided you can protect them from being covered with snow/ice. If it were me, I'd probably place them on a sheet of plywood (or 2x4's), connect the trickle charger and then put a sturdy box or a plastic storage bin over them, just to keep them from being exposed to the wetness from melting ice/snow through the winter.