upgrading to 4 6volts sounds like the right idea for your use. I'm surprised a living quarters race trailer with 12 volt systems (lights and water pump) doesn't already have a converter so you might want to look around and see if it is already there and working
If I were you with long of a trailer and its use do the following
- Install the 4 6volt batteries as you propose
- Find and fix the existing converter or install a new one
- Keep the 12v in the back to operating the winch and door but use an automatic battery isolator to seperate the 6volt bank from it when not charging via generator or shore power
