My 2008 Tiffin FRED chassis did not have them. I first replaced the shocks to Koni's in 2011. Then last summer I added Sumo springs to all four corners. They were only about $190 for a pair.
They are like a large marshmallow that goes between leaf spring and frame and is tall enough to have some weight resting on it all the time. As your suspension compresses, whatever the material is made out of gets tougher the more it is squished. So, if you were hitting the hard bump stops (like I was), the Sumo springs made a huge difference in ride quality. More of a cushion than a hard stop. They also helped with side to side sway.
So, for me, they were well worth the $400. But, I would not pay $800 or $1,200!
Good luck.
Jim and Deanna
2008 Tiffin Allegro 35QBA FRED
2007 Carson Trailer 22' Titan TH
Trailer Toad
Me, Wife, Boy/22, Boy/19, Girl/17
1985 Toyota 4Runner
TWO quads, THREE kids, TWO motorcycles, ONE wife, TWO dogs, ONE cat, TWO Polaris RZR's