You can get a replacement breaker from Onan. Pricey, but it's the only one that fits. It's a twin 35A breaker, as Koop describes. I think about $80-$100, depending on the source.
It could conceivably be a bad breaker, but I would be checking the loads carefully before concluding that. The last thing you turned on in merely the straw that broke the camel's back. It's the total aggregate load on one leg that will cause a trip. You might also have a short circuit somewhere - that will always trip a breaker.
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL