If I’m parked up in the sun the inside of the van can get pretty warm, although at floor level the battery compartment seems to stay a bit cooler. Anyway, is a battery temp sensor a useful thing for summer temps, or is it more for charging when temps might be getting really low in winter?

I’ve got a victron smart solar charger, and they make a temp sensor that connects to it to ensure its charging at optimum level

Mine has one. Allows faster charging of the batteries in cold weather, and reduces charge rate to prevent them overheating in hot weather