We were towing a rental travel trailer (~3500 Ib), going east bound on highway 80 near Martinez, CA. The travel trailer suddenly went into oscillation, the amplitude got larger and larger, and finally both SUV and trailer landed on the sides on the embankment. The SUV was supposedly in "tow" mode, so the feature "Trailer Stabilization Control" should have been activated to reduce the sway/oscillation (and perhaps issued some warnings). However, we only got traction control alarm just prior to the crash.