If the cabinet that goes across the back of the trailer is made of fiberglass, those round head nuts are acorn nuts. They are used as decoration and backers for long pop rivets. So to take the cabinet down you have to remove the outside caps and drill out the poprivets. If you have an ensolite liner you can reinstall with 3/16 x 5/8 poprivets from a Hwd store and reuse your castle nuts. If you have rat fur lining you will have to get longer pop rivets from Scamp
. Or you can reinstall with stainless screws and nuts.