Tim,
If you are going to drain and refill the transmission, and you just need to add a gallon or so of fluid to drive the coach to a shop, any Dexron III transmission fluid will work. But if you are only a gallon low, a short drive to the shop will not cause any problems. Transmission dip sticks on these coaches are not always accurate, so that is the reason that I recommend checking the transmission fluid level with the shift pad on a regular basis (at least monthly). However, like Steve said, I strongly recommend using TranSynd or an equivalent fluid when you service the transmission (drain, change filters, and refill).
Gerald