My trailer was built by
West Coast Trailers, in Central California.
Wells advertises plywood walls because they are selling you a cargo trailer. Everything from there is an addition.
All interior walls must be covered in a smooth, non porous material, such as frp or stainless.
I would go with a manufacturer that only builds kitchens. They are the experts.
Do you know what you are going to serve yet?
Your menu is going to dictate what equipment you will need and how it is laid out.
You also need them to understand your health dept regulations and any other state or local agency that may have a say so in your kitchen.
Washington state has some of the strictest guidelines on mobile kitchens, a stock trailer built for anywhere else would not pass inspection here.
I had to have a 4th sink installed for produce washing, no a/c is allowed because it is not energy efficient, all electrical conduit must be exposed and not run inside the walls.
The serving window must be tempered glass, propane bottles installed on the tongue only, not on either side on platforms.
This eliminates installing a generator on the tongue.
Waste tank 35% larger than fresh water.