I know this is going to sound weird but have you had the alignment checked on the car? The rear end my be out of alignment. Are you 100% sure it was drifting or could you have not been straight from pulling over? I would take it to a parking lot or a road that is not busy and drive directly on some straight painted lines and then stop and get out and look.With the long tail swing of a MH it don't take much to move the tongue of the tow dolly one way or another and make it appear that the toad is not straight behind the motor home.
