All six of our batteries are outside in a similar position. Freezing is not an issue for lead acid as long as you keep them charged.
Our batteries are in sliding trays, this makes watering easy, I suggest you make sure you have access to your leisure batteries for that purpose.
If its two batteries series connected, (example, 24V system ?) and "isolator" means bigazz master disconnect switch.
Yes, it can be fitted into the jumper lead between batteries.