What type of trailer...is it a pop-up, hybrid or regular trailer? Is it 21' overall, or is just the camper box 21' long. I think your answers will vary with the answer to these questions.
The largest unit I ever pulled without any sway control was a Starcraft Starstream that had a 12 foot box....so around 15-16 foot overall length. I never had any sway, so I never even thought about sway control, and when I bought it, the RV dealer never said anything about sway control.
With a 21 foot pull behind trailer (full heighth), I really don't know what is typically recommended, but some form of sway control would be "cheap insurance" if you plan to tow it with a smaller, low profile vehicle. But...I really can't say I have an answer for you, but I'll be interested in hearing what others have to say!!