I think I depends on your van ... our 2009 Expanda required the battery switch to be turned on in order to use and/or charge the battery, the switch basically isolated the battery from all activity. The new van with solar and a Drifter panel apparently charges wherever the battery switch points.
But either way the Setec is a smart charger and will help to keep your battery conditioned and extend it life .... so it likes to be on all the time when connected to mains. The only time you should probably turn it off will be to help stop any drain on the battery if the vans in storage and not connected to mains ...