Thanks, Shrinking V.
I hadn't read it, so was remembering a brief skimming of the book jacket and catalogue write-up. In England we'd call it a gypsy caravan, and caravan (in my mind and John's) could translate to the North American trailer, thus losing the gypsy aspect in the translation. That ending is great. Dahl's a bit funny that way though, he seems to have such empathy for children, but was a bit useless as a dad apparently.