300 lbs hanging behind the rear bumper is going to take around 200 lbs off the tongue weight. I would think if the tongue drops below 11-12% of the trailer weight there will be sway issues. The trailer is a big lever with the forum at the axle.
I have always thought the weight limit issue of 100 lbs by TM for the bike rack was a weight and balance thing and not a mechanical limit of the frame/hitch???
See the youtube trailer sway
youtube sway