Streamlined polished aluminum with a backdoor, bathroom, bedroom, dual air conditioners, portholes, tail fins, vestigial wing buds, superfluous teardrop running lights, TV and a visor all pulled by a sleek Hudson Hornet. The ideal trailer might still need flared fenders and a rotating gun turret.
Pre-interstate Highway travel required that you slow down and drive through towns. You got to see their Main Street Christmas decorations, their city park, and their places of business. Restaurants and motels were located in town rather than clustered around interchanges.
Because we brought our own food and pulled a kitchen, I can only remember a handful of times we stopped at a restaurant of any kind. Most of those occurred after we got rid of some of the older girls. Dad claimed he didn't like soft ice cream so even the Little America 10 cent cones were highly unlikely. Cars were not dining rooms on wheels. Cup holders consisted of two shallow circular impressions in the sheet metal of the inside of the jockey box door. They were more designated resting areas than holders.
The future was streamlined by industrial designers like Henry Dreyfuss, Raymond Loewy, and Norman Bel Geddes. It was fully realized in my world in toys.