George, (Fresno, CA)
George, I’m going to assume you are referring to a stand-alone battery charger that is 12-volts only. I mention this because some shop chargers are equipped with a 6-volt output option as well as 12-volts DC. Since all DC components on RVs operate on 12-volts, (water pump, fans, lamps, etc.), your two, 6-volt batteries, as installed, are still set up to produce a 12-volt final output. The only difference is the connection between the two batteries.
At the basic level, two 6-volt batteries are wired in “series” and two or more 12-volt batteries are wired in “parallel” in order to produce 12-volts final output. Take a look at the diagrams here; two, 6-volt batteries above and two, 12-volt batteries below. The final output that provides battery power to the components in the RV is still 12-volts DC. Therefore, both configurations can be charged by any 12-volt battery charger by simply connecting the charger’s red cable clamp to the “plus” 12-volt terminal lead and the black or ground cable clamp to the negative terminal as shown here.
##RVT808