When did slide outs first show up?

One of my best friends family used to travel with the father to pipeline construction sites in the summers of the 60's. They would buy a 35-40 footer each year for their summer trip and sell it just before heading home to south Lousiana. Those would be called Park Models today.

In 66, they showed up on their way north with a trailer with a tip-out dinette. In 67, they had one with three tip-outs.