If they are two batteries mounted in SERIES then they are two each 6VDC BATTERIES. If they are wired together in parallel then they are two each 12VDC batteries...
Everyone should have a multimter to measure such things...
Go pick up one from HARBOR FRIEGHT. Usually free when you purchase something from them...
I would take a CELL PHONE photo of how they are wired up before you remove them...
You also need to learn what made them run down... Your new ones will do the same thing...
Hopefully you have a smart mode converter/charger system with your RV that uses the multiple DC VOLTAGES to keep your batteries re-charged when connected to shoe power or running from your generator.
Some of the older RV UNITS only have single mode converter/chargers and this 13.6VDC only charging voltage may boil out your battery fluids over time which ruins then eventually.
When parked you should have a battery disconnect method to keep the parasitic drains in your RV from draining down the batteries...
Roy Ken