In the classic games they were these caravan wagons that people made from old cars and they were pretty common too but after fallout 2 they did not appeared anywhere else
It's a cool idea, but I can see them dying out as things changed. Rubber isn't native to North America, and synthetic rubber requires a whole industry to create. As the tires decay and fall away, the hubs are small, metal, and not a particular advantage in effort over strapping the goods on a Brahmin.