Originally Posted by sknight
If you run one or the other I doubt you'll see much change in fuel economy.
Could always run the engine driven AC normally then fire up the genset and roof AC in hilly areas as needed. Might be the best compromise.
That's why I suggested he turn off the dash air and use the roof air in those "mountain conditions".
Years ago I had a Chevrolet powered motor home, and running that dash air or not, made a very noticeable difference in how the engine performed.
No idea on which is really cheaper to run, but lessening the load on the engine is enough reason for me to shut off the dash air.