adding 6 volt batteries to my 5er

I have 4x GC2 driving a Prosine 1800 for big stuff and a GoPower 300w for small stuff.

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

I have 2 6 volt batteries and a 1000w pure sine wave inverter that runs our residential fridge while going down the road. The batteries will take care of it all day with very little loss of charge. Of course, the charging line from the truck is helping. Our inverter only supplies the fridge but could be used to run my small air compressor or something else if needed. We don't dry camp so it handles what we need.