Thanks everyone for your responses. I was hoping there was something that I was missing.
I do plan on adding additional batteries in the future. I have my Onan gas tank beside the batteries so my options for them seem to be limited.
I think given their location, I will definitely look for batteries that are sealed.
It also seems to be me that when I have 4 batteries, wiring the batteries up serially and/ or in parallel would be problematic given the location and space available.