Dena,
I wish that I could give you an easy answer to your question, but since I do not know what the "diesel mechanic" did to your coach, it is impossible to trouble shoot your problem with out much more information. You just need a qualified person to thoroughly diagnose your coach before they just start throwing parts at it. A qualified mechanic would not have installed the wrong alternator or any alternator until they knew what was wrong or sent you out with the original problem still present. Since the Santiam is an Indiana built coach, I am not familiar with the charging system, does it use a battery isolator or a "BIRD" system?
Gerald