I hauled a 24ft travel trailer all over the mountains of Idaho with my 2008 F-150 without a WD hitch..Never an issue or even close.The trailer had breaks and I had a controller.
If the trailer has breaks,get a controller.For a 21ft trailer and a heavy duty 1/2 ton a WD hitch is not necessary in my opinion having hauled several in that length.
I used 24ft as the max length without a WD hitch based solely on my experiences towing trailers.I have towed up to 41ft without one but it was a heavily modified 3/4 ton.
You will know if you need either the first time you take it out.
