Creativepart I'd like to thank you for your advice. With your help I figured out it was the inverter or the on/off switch inside the coach. Per your advice I called Magnum and like you said the guy that answered was very knowledgeable. He said on the back of the on/off switch (interior of coach) there should be a phone jack and a spade terminal. I told him I only saw a phone jack.
So I got home and looked at the back of the on/off interior switch:
I was thinking the whole day (I was at work when I called Magnum) it would be a huge job to run a new power supply to the on/off switch. However, I was shocked when I shined a flashlight into the wall and saw the missing spade terminal. I fished it out with a coat hanger and reconnected it and now the inverter works :-)
Again, thank you very much for your help and as with most things an easy fix once you figure out the problem.