Most RVs come with two or more batteries from the manufacturer.

One battery will be configured as the starting source for the engine.
(Chassis or engine Battery)

The second battery will be configured to run what people refer to as
the house. (everything behind the driver seat).

By default, the engine battery charges off the alternator anytime the
engine is running. Older RVs isolated the house battery and the
engine battery with a heavy relay. The heavy relay engaged when the
engine was running or when a switch was thrown. This was a fail safe
so if you were camping and using your 12v accessory, you wouldn't run
the engine battery down, but while traveling, both batteries could
charge off the alternator.

Most new RVs have what is called an Isolator connecting the two
batteries. The isolators are a bit more efficient at making sure both
batteries get charged, but never drain the engine battery when parked.

Its not uncommon for people to replace their house battery with two 6
volt batteries such as what it sounds like you have done. This set of
batteries is what you will want your solar panels to charge.