Mary - the trailer occilations behind the motor home are caused by vortex shedding in the slipstream behind the motorhome.

A cool movie of the flow behind a square body (=motorhome) can be found here.

Aluminum trailers are particularly susceptible because, in addition to being light, they are flexible. When the tongue flexes too much, too many times, it breaks from fatigue.

The cure is to make the tongue stiffer. Trailex has done this over the years, but I attach my sailbox to the toungue, making it a structual part of the trailer to help stiffen it up.