Welcome to iRV2.
Is your coach is on a Ford chassis , or Workhorse ?
Both manufactures used both sizes of rims , Workhorse on those of lower GVWR, under 20,000 I believe . Ford used the smaller wheel up to a certain year.
Is the serial number on the tire size decal correct for your chassis?
Wouldn't be the first time a decal was installed in the wrong coach.
As others have said you need to check the tire's age and load weight rating.
As far as the rough ride is concerned , your need to weigh the coach; 4 corner is best ; loaded for travel and set your tire pressures according to the tire manufacturers inflation chart . The inflation chart is available on line.