The first Monocoque body car was the 1934 Citroen Traction Avant. It was also Front wheel drive with full four wheel independent suspension. America didnt catch on for many many years.
In terms of ultimate strength a Box frame will be stronger than the old "C" section frames.
The issue isnt really frame strength. The issue became making cars lighter, stiffer (for better handling) and better fuel economy.
One of the major reasons people did not tow with the pickup was there was no where to put the family. Thats why the station wagon was so popular. And the station wagon had been around since the 30's in one incarnation or another but really dates to the 40's. But it wasnt until after WWII that Trailering really took off and that meant using the family car. Few had the resources to have two cars.