A 30' TT behind a half ton provides a lot of surface area for the wind. Glad you survived the incident.
Had it happen to me once. First I saw the entire trailer in the left mirror, then the entire trailer in the right mirror, then left, then right, etc. Used two lanes and left squiggly black lines on the pavement from the TT tires sliding sideways. Fortunately we didn't crash although I thought it was imminent.
To anyone faced with this situation, the correct response is to maintain throttle and manually activate the TT brakes.