My hesitation with Lithium batteries isn't based on fire. They also have a minimum temperature they should be exposed too. I'm sure it varies but the ones I've looked at had an absolute minimum temperature that was above what it often gets to on cold winter nights. If my camper is sitting outside all winter that would mean needing to unhook the batteries and bring them inside. With AGM or flooded batteries, as long as they are charged (and the solar will take care of that) there's no reason to unhook them and bring them inside during the winter.