The grooves make the aluminum siding stiffer for a given thickness of material, and thinner siding is lighter.
Manufacturers did this also with other materials, including fiberglass before it became practice to laminate the skin to a core, so that the core material stiffened the wall.
There are a lot of different RV manufacturing practices, tradeoffs among cost of materials, labor, and capital equipment for labor-saving and time-saving methods. There are also tradeoffs of strength versus weight.
There are TTs with molded shells, wood frames, aluminum frames. Framed walls can be covered with aluminum or plastic skins, with either framing material. Fiberglass skins can be smooth or have ridges.
Many RVs are built today with no wall framing, rather the skin is laminated to a core, the skin can be aluminum or fiberglass or some other composite, and will most likely be smooth, though when Winnebago first developed the technology they used a corrugated skin. Cores can be different foam materials, or honeycomb or corrugated; foams are popular for insulation.
There are enough options that you don't have to buy ridgey outside walls. Most people will argue that their choice of construction technique is the best, even if they don't understand how their RV was made and believe it to be something different that what it really is, because the salesman convinced them.
