I have never found a reason to ever turn the switch off.
As for charging while driving. The alternator will only charge until it has topped off the TV batteries. Yes during that time the trailer batteries will see a charging voltage but once the TV batteries are fully charged the alternator will drop off. Do not rely on the alternator as the only means of charging the trailer batteries.
It generally takes about 20 miles of travel for an alternator to recharge a battery from a single start up. Once the TV battery is charged and the alternator drops off there will be current flowing to the trailer battery but at a greatly reduced voltage.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles