Nigel,
My first experience with a "band" trailer, in 1965, was a similar encounter. We were traveling from our hometown, near Tulsa, OK., and returning from a gig a hundred miles or so into Kansas. All our gear including drums, amps, Bogen P.A. (anybody remember those?} and my Hammond B3 were in the trailer.
Somebody looked out the window to see the trailer passing us on the highway. It as 3 a.m. and there was no traffic. The trailer gradually coasted to a stop some half-mile down the road and gently rolled into a wheat field. It didn't even tip over, and all the gear was secure!
I'm sure we didn't fasten it properly, but that was nearly 40 years ago!
DonM