Since there is room for only two batteries, consider moving to 12 volt jars. AGM would be my choice.
By doubling the number of cells, the load per cell is reduced to 50%. It would be important to use thick wire and balance the wiring.
Many rv'ers use a small inverter for little loads. Best bang for the buck is a Victron 350 watt.
This is what is balanced and best for twin twelve volt batteries.
As it often doesn't cost a dime more to do this, I think it is worth the trouble.
If you wish to understand the "why" surf here:
correctly interconnecting multiple twelve volt batteriesOthers may say it doesn't matter--but unless there is a compelling reason to not optimize charging and discharging why not do it the best possible way?