As stated before "it depends".
There are some brands that come from the factory with them already installed,Fleetwood products such as my 2006 Tioga come from the factory with a
BIRD a Bi-Directional Isolator Relay Delay
It Looks like this
Check in your battery compartment, that is where mine is
Click For Full-Size Image.
It charges both batteries when either one is
being charged. When the coach is being driven,both
batteries will be charged from the engine's
alternator. When the coach is plugged into shore
power,both batteries will be charged from the
converter. If neither battery is being charged, the
batteries are fully isolated. The controller also
senses heavy loads on either battery to prevent
the wrong battery from being inadvertently
discharged.
While sitting even while plugged in, if some type of charger is not hooked up to the Chassis battery it will go dead from the parasitic loads.
I keep mine plugged in all the time at home and with the BIRD both Chassis and House batteries stay charged.