On most aluminum sided trailers the bottom trim is one piece, end to end. It is usually about 6"-8" high with a nice rolled under bottom. The main purpose is to hide the side of frame.
Is this formed by a machine at the RV factory or is it pre-formed ?
It`s pre-Formed -- When installing the siding, you start at the top & measure so the siding will come out on the bottom so the pre formed piece will fit -- Worked in TT factories for years & some of the time was installing metal siding -- Bill Willard
years back my dealer, a mom pop shop had a machine they could set the dyes and make you whatever design you wanted, made me a huge piece for my truck camper fit very nice.