Update for anyone interested.
As mine had a 24 series battery on it I just bought another battery box and another matching group 24 battery. By cutting the handles off one end of the lower battery boxes there was enough room in the existing tray to mount both batteries. I cut part of the lid off one battery and installed the second battery in the opposite orientation and was able to get both lids on. Jumper cables were purchased at NAPA for $10 each all made up with terminal ends on them.
I have also placed a disconnect switch on the negative lead to the trailer to shut it all off so the parasitic loads will not drain the batteries until they are needed.
So I now have double the amp hours of the original setup.