Tons of advice already on charging. I have also heard that many converters are not optimal battery chargers. I make sure my toggle switch is in charge mode when I am driving to and from the camp sight so the engine's alternator does the charging.
As for adding a battery, I added one to my 1990 Winnebago Warrior. It only had room for a small house battery up under the hood. So I added one in the storage bay right behind the generator. I purchased a long set of jumper cables and used it to wire back to the original battery. It was cheaper than buying the same gauge wire and building my own cable.
Steve
Steve