Help I'm new to campervan life and don't know how to use a leisure battery.

I knew a woman called Freda once. Pray tell us,(maybe with a pic) what a Ford Freda is? ta.
Normally the charging is split between the leisure and the car battery.
I'm stabbing a wild guess that the switch is...if you switch it on, it will start to charge the leisure battery.?? (shared with the motor battery).

if you don't have a multimeter then get one its going to be really useful figuring out whats going on with electrics.
in this case, hook the multimeter to read the voltage of the leisure battery. now start the van. if the meter is reading 13 volt or above then charge is been pushed to it. now flip the switch, if the voltage drops to 12v(ish) and below (or visa versa) then that switch controls the split charge. if nothing happens then either that switch controls some thing else, or its broken.

as for how the swapping about goes. most systems be they simple relays or posh multi step chargers the swap is automatic. If the engine is running then then the leisure battery is charging, turn it off and the leisure battery is isolated from the vehicle starter battery to you can't flatten your means of getting home. some people add a switch inline with the wire that signals the relay to close (not the main feed between the batteries) so they can have more control over the charging and prevent the leisure battery been charged if it is know to already be full.