I added a Delta Box. This model is made for the A frame of a trailer.

I added 3 additional deep cycle batteries which gave a total of 4. I stayed with 12 volt because my original battery was brand new and did not want to just throw it away. Plus I bought my deep cycle batteries at Costco and the 12's had a two year warranty and the 6' had one year.. If I was to replace them later I would have went with 6's. It would be about 340ah vs 440ah.
Note: After taking this photo I found out that I had it wired wrong. I moved the negative battery cable to the other end of the battery bank for more even use. You want to think of the bank as one big battery.

I vented the back and bottom of the box

I made a shelf that sits above the batteries that I used for storage of power cables, water hoses and tow bars...

You want to consider how much weight your adding to the tongue. My trailer was a heavy toy hauler and the tongue was designed to hold a generator where I put the battery box.