adding 6 volt batteries to my 5er

You can do anything you want with batteries. How you do it will determine the outcome. I would suggest you create an independent bank and run a large inverter off it. The six volt batteries like to be worked hard. They like fast hard discharge and hot heavy charges. The typical RV doesn’t have the loads to work LA six volt batteries. The chargers are not up to the job also. Just take a look at any commercial golf cart, floor washer chargers and you will quickly see the differences. They are charging fast and adding water.
MM49