The evolution of the trailer coach began during the years of the Great Depression. Rigs of the time were 8 feet in width, a standard that would be used into the 1950s. Everything in a 1930s unit was built-in. Seating and kitchen areas were stationary, as were sleeping accommodations.
Two 1930s trailer coach adverts. The first shows a 1934 Federal rig, the second, a 1938 Kozy Coach. At the time, the typical unit measured 16 feet in length and did not have a complete bathroom. This feature would not appear until the early 1940s and did not become standard until the early 1950s.
Adverts from http://www.allmanufacturedhomes.com (Atlas Mobile Home Museum)