I'm a fan a sumo springs. They are a great supplement for supporting a cargo load.
They are not however, a fix for sagging or overloaded leaf springs. If might not hurt to weight your coach first, to get a baseline. If you are at or over GAWR, you might need to add a leaf first. They probably won't do anything for getting blown around (sway bar and shocks help more with this), but they can improve the loaded ride of you coach.
And it's a good idea to weight anyways, to make sure you aren't overloaded on the rear tires. "28 foot" models tend to ride on short wheelbases compared to their overall length, so diligent loading can be really important.
