[Carl n Susan]

As you aren't boon docking, another deep cycle battery should be adequate if you have 30/50 amp shore power to keep it charged via the converter. A separate battery charger usually isn't needed. My choice would be two 6V golf cart batteries (Costco or Sam's Club have the best prices), wired in series, to provide more than enough battery power.

When you get/have shore power, whether 30 or 50 amp (off a pole or even the generator), your existing converter will be fine to keep the batteries charged.