Larry,
The only advantage that I see to synthetic engine oil in your coach is that it will very slightly reduce engine friction and therefore give you a very slight increase in fuel mileage, however it will not save you any where near the added expense of the increased oil cost.
Synthetic oil does not increase your oil change interval according to Cat, and even though it may increase engine life expectancy marginally, the engine in your coach should run 6 to 8 hundred thousand miles on conventional oil. So why would you want to extend the engine life longer than the 240 years it has at you current use rate.
In other words, I think that synthetic oil in your engine would be a waste of money.
Gerald