I finally got around to adding a second battery to my fifth wheel. The last trip we took was dry camping and the original 12v marine battery died on us, so I converted to two 6v golf cart batteries. It was hard finding a battery box that fit, but I was able to locate one from a company on the Internet. I had to cut holes in the box since I couldn't find one already vented. I cut a hole in the cover and modified a large sprinkler pipe to accept the vent tube. I then cut a hole on the bottom of the box for drainage. It took a few hours but it was worth it. We also put a mor/ryde pin box while we were upgrading stuff. Can't wait for the first trip this year!
I have added pics below.
Mor/ryde pin box