Sorry, I can't offer much help with this. My trailer came with one battery
but space for two. No cover, the batteries are housed in separate battery
boxes. These battery
boxes are readily available and easy to install.
I've tried both the two 6 volt batteries wired in series and the two 12 volt batteries wired in parallel arrangements and found little difference in performance.
As an example, two 220 amp/hr 6 volt batteries wired in series yields 220 amp/hr of 12 volt power.
Two 110 amp/hr 12 volt batteries wired in parallel also yields 220 amp/hr of 12 volt power.
The 6 volt batteries must be evenly matched. If one fails, both have to be replaced.
The 12 volt batteries can be used separately. If one fails, the other one is still usable so you only have to replace one.
I used both for 7 plus years before their performance dropped.
I recall a situation where the breakaway switch on my trailer was "accidentally" activated. I awoke the next morning to my "low battery" alarm as the single 12 volt battery I was using had been discharged by the electric brake being activated all night.
Had I been using two 6 volt batteries in series (or both 12 volt batteries in parallel) I would have had no power, not good for boondocking
in the back country.
Using one 12 volt battery, I had the other one to use while my solar
panels recharged the other one. This was the deciding factor for me.