1. Someone miswired and those wires should be 12 volt.
2. Some RV's have LOW and HIGH 12 volt lights in the RV (non LED). So, they use a 6 volt source to power the interior lights that are connected to make the lights "DIM". You usually have a 3 position switch for the affected lights. OFF--Then up supplies 12 volts to the lights and Down supplies that 6 volts.
3. It is VERY rare to have 2- 6 volt batteries on a Towable. So, I would bet the 12 volt original batteries were replaced with 2- 6 volts and those 2 wires not connected to the proper 12 volt terminals. Doug