Based on your description of the fan running so much I would suspect you may have a bad battery. The converter is working VERY hard to charge it. That could very well be what toasted the first converter.
As far as dealer looking for a cause of the first failed converter it is not likely they will go any further than fixing the obvious problem. Get it out the door and move on to the next job. Most RV dealer are not well versed in electrical issues.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.