I laughed when I read this. I bet I know what it was for! :C
I built a shelf for my MH that had three legs!!
I needed a shelf 'along side the bed'. On most MH's the built in nightstand along side of the bed has a slant coming off the bottom.
To put a shelf there along side the bed you can't get in 4 legs. I used three offset maybe your previous owner did it with just two.
I used it for a place for my cat to ride safely when traveling. See the small part of the edge of the plywood resting on the edge of the bed?
I painted my plywood to match the MH colors. AND I used to store it under the bed when not in use. :W