These roadside diner designs were found in patents from the 1920s, 30s, 40s and 50s.
Sterling Streamliner diner A design for the famous Sterling Streamliner diners. Sterling bought the design from Roland L. Stickney in 1939. Several Streamliners are still in use and examples can be seen here and here. A Streamliner now sitting in Rhode Island was the first diner to be listed on the National Register of Historic Places. 1939
Valentine Diner Valentine Industries was known for its prefabricated diners. The patent describes this as a "Roadside Shop," though it's pretty clear it would be used as a small restaurant. This design could have been easily transported on the bed of a truck, like this. 1947
Roadside diner with furniture This design for a roadside diner is unique in showing the interior layout. Another Bertron G. Harley diner design can be seen here. Classic roadside diners were pre-fabricated and shipped to the restaurant site with furniture already installed. 1938
Ward & Dickinson Diner In the 1920s and 30s the Ward & Dickinson Co. built diner cars and sold them throughout the Great Lakes Region. A version of the diner body shown in this patent is still in operation as the Miss Port Henry Diner in Port Henry, N.Y. 1927
Diner design A diner by Arthur Sieber, who also designed the White Mana that debuted at the 1939 New York World's Fair as the "diner of the future." 1938