If you want to get a full understanding of what is going on, I suggest you do a search for a study done in England about Trailer Snaking (that's what they call trailer sway - the oscillation you are feeling when passed by a semi) -
THE DYNAMICS OF TOWED VEHICLES
An experimental study into caravan snaking
Christopher J Killer
This study is very involved and has lots of math. However, skimming thru will get you to easily understandable conclusions.
Also, check out a number of RV forums. IRV2, Airstream. etc. There is a wealth of information on just about every hitch available. Note that most people are adamant supporters of what they own...
I will agree that doing something to control sway is a good thing. What you invest in will depend on:
How much risk you will accept
How much you are willing to spend to control the risk.
I have made my decision on what I will be using, send me a PM if you want my decision and reasoning.