Rocky,
It sounds like you are misreading the manual. I think that the hydraulic system on your coach that runs the radiator fan and power steering uses HD 46 oil, but if I am not mistaken your hydraulic leveling system uses Dexron III. Also, most of the Thunders in your era had two electric hydraulic pumps located in the front of the coach, one ran the HWH slides and the other one runs the Power Gear leveling system. If yours is so equipped, be sure you are adding fluid the right pump. Also, you can find HD 46 oil at most HD equipment or farm supply stores (Tractor Supply is the most common) if you need it.
Gerald