Since we seem to be going for the newer models, the GM 8.1L engine with the 6 speed double overdrive Allison transmission will get much better mileage than the Ford.
Basically both engines are fine and will outlast any coach that they are installed in, with normal care. The Allison transmission is a proven component and is known to be bulletproof. Both engines really get low mileage so if that is your criteria for buying a motor home, you will be disappointed.
Look for the floor plan that suits you best, and do not worry about the engine. Both the GM and the Ford will get you there and back.