I can usually diagnose a lot of issues my coach is having, but when I can't, I do a process of elimination and try to determine what part is most likely causing my issue. I then take the price of the part and weigh it against what it would cost me to have someone work on it. In your situation, from information you gave us, it sounds like a bad ignition switch. My guess would be the switch is somewhere around $50.00 or less (usually made by Ford or Chevy). That would be substantially less than a $200.00 minimum service charge for a shop to look at it and most likely replace the switch anyway. My decision would be to replace the switch myself and if that wasn't it, I would keep it as a spare.
The above process serves me well 95% of the time.