Green Bay Pam & Rick

Well I've reach the end of my trouble shooting experience. Our Onan generator is having issues with half of our electrical system. The generator runs fine as long as the bottom breaker on the generator is in the off position. Today we had an RV tech out that is certified Onan service and he did an excellent job checking out the generator and found it to be operating as it should. We determined that the left hand side of the breaker panel has a mystery situation going on. Everything works on shore power, but the generator shuts down within seconds if we try to close in the bottom circuit. He even switched the breakers to make sure the breaker wasn't the problem. Head scratcher! Anyone had this problem? Any guidance would be greatly appreciated. Thanks!

Rick