I advise to bring the batteries inside if storing the motor home outside off-site in extreme cold temps where people are easily tempted to leave their rig unattended through the winter season. Batteries left in the RV hooked up and ready to go, there is a small continuous power drain, both on the chassis battery as well as the house batteries. Once completely drained, they are suseptible to freezing. If they freeze, they are permanently damaged.
If the owners are certian all batteries are fully charged at the time they park their rig off-site for many winter months, they could simply unhook the ground connections. That eliminates the small continuous power drain, giving them a much better chance of surviving the winter. But no guarrentee. So I blanketly state, it's best to bring the batteries inside.
If the owners live in a less severe climate where winters will see freezing temps, but never get below the 20 degree zone, they could get by with unhooking the grounds and leave the fully charged batteries in with the rig. The climate where they live is the main variation, but a blanket question yields a blanket answer. Many people get lost when I get into such detail, so I try to keep it simple.
I live in the Chicago area and volunteer in a ministry HERE
where cars are donated. We have a lot in back, sometimes having hundreds of cars parked. Many batteries were checked to be fine, later to be dead if still hooked up and unattended over the winter.