Do you drive 100k a year? Synthetic oils are great for the high mileage drivers, because of their long term durability. Synthetics have no better lubricating properties than that of convential oils. In most motorhome applications with an annual mileage of 20k or less per year, synthetics are simply a waste of money. Change your differential oil every 24 months. Moisture, not wear is your biggest culprit.
Transmissions, differentials and engines like clean oil, and for my piece of mind, oil that is changed regularly.