Real cause of "rivering" or cupping? Air pressure?

our right front tire cups on our '04 Country Coach DP. Tampa Spring in Tampa and Ocala FL told us that we have to set the "toe-in" on our coach every year or two. Since we have been doing so, no more cupping on the right front. We also changed from Toyo tires to B F Goodrich ST230's load range H. They are wearing much better than the original Toyos.