Buy those plastic office drawers at Walmart, the 3-high kind. Of course, our Uhaul
is the larger one, but we use two of those tucked under the front dinette. In our Burro
, the PO had hung a set of those shelves that are made of cloth, like shoe holders, but not the envelope kind of shoe holders. These were shelves made of cloth in a column. I have seen them at a Container Store. They worked well, although I also bought some Coleman net bags to hold the stuff inside them and made velcro holders to keep the stuff inside the shelves.
Some people with smaller trailers build their porta-potties under the front seat.
If you search this site, you will find pix and ideas from lots of 13' owners that will help you. You do have the smooth walls, but we prefer those. We use vacuum shelves, hooks, and mirror on those walls.