FORD itself made only a couple wheelbases. I think most RV companies order the 158" WB although there's another (176" I believe). The RV companies then have the chassis "stretched" by a specialty company, or maybe some do it themselves.
For example, Our E450/Jayco has a Ford label saying 158" but Jayco has a label saying 218" so the specialty company (Sellars in our case) extended the WB by 60" (Five Feet!). They also extended the rear overhang. Ford has an engineering standard for doing this. If followed, the chassis retains its original capacity.