FWIW I'm driving an '00 E250 raised roof (12") cargo/work van as a daily driver, 7800# typical gross weight on Michelin LTX LT245/R75-16's @ 70psi and Bilstein shocks, Hellwig sway bars front & rear. urethane bushings everywhere.
The rear sway bar was installed very soon after buying this van and its difference was huge. In fact the first freeway excursion after installation some cell-phone talking texting azzhat swerved in front me requiring a huge evasive to the left lane change on totally dry roads. Already familiar with the whip effect this might create I was "up on the wheel" trying to sense and anticipate the van's movements in order to avoid some catastrophe. Since I simply had to move over one lane albeit rapidly the rear sway bar "caught" the noticeable body roll, counteracting it immediately (to its limits) allowing almost no rear end swerving or fishtailing. Steering wheel input after the lane change was minimal----a huge breath of relief was the only sound I heard.
Had this situation required an additional move to the right immediately after the left move I can't be sure what would have happened however the sure-footedness I felt in my seat suggests it would have been manageable, without much more than a massive pucker factor being induced.
Having driven similar empty standard E250's I was pleasantly shocked at how well the rear sway bar preformed. Without that rear bar this maneuver might have been a lot more frightful. At the time the stock front bar, no-name shocks and springs in general were quite worn (eventually replaced) so to my mind this was the perfect situation to assess a rear sway bar.
I looked into the Roadmaster products, talked with a few in the company and finally concluded they wouldn't be a huge improvement over my current set-up. They did offer I could discard my Hellwig rear bar however we never discussed running both items simultaneously. I was enticed to give Roadmaster a trial since they have a great unconditional satisfaction guarantee----the work entailed in switching them out maybe twice didn't appeal to me however. I stayed with the Hellwig, front and rear.
Long story short if your current vehicle has no rear anti-roll bars at all and/or your front bar is old or undersized due conversions that add significant weight to a standard chassis improving these aspects greatly enhances load control. Of course that's assuming your suspension including coil or leaf springs are in good order and sufficient to carry your daily driven weight. RAS is one way to go for the rear but for me the Hellwig products are best. Its also important to have a "system" of sway or load control, integrating your components to work together. Obviously anything done to the front or rear alone can significantly affect the other end which is vital to keep in mind.
Hope this wasn't too off topic or boring----just that I've been there, done that and this is my reflection on it all so far.