What is it called?

Let me see if I can describe this. Under the front of the 5th wheel, on each side there is a rail with screws attaching the bottom and sides together. What is the plastic/vinyl cover that goes into this rail. Went to research it and realized I didn't even know what the name of it is.

Kinda hard to get any pricing if you don't know what it's called. Any takers on the answer?