What would you recomend for a transmission that shifts erraticaly when pulling a trailer?

2009 Ford F150 with 5.4 Litre V8 - When I pull my 7500 lb travel trailer the transmission will downshift on a flat road at highway speed for no apparent reason. It only started happeneing the third season of use. The dealership gets no error codes on the diagnostic but during a test drive agree there is a problem. The transimission has been re-calibrated 3 times.