Converter issue

Lets keep this simple. He has burned out 2 Converters and on his 3rd. IF he had a voltage supply problem(open neutral/loose wires), OTHER appliances(TV/Microwave/Refer 120 side fuses/Fireplace) would also fail. I would suspect bad battery or a miswires(reversed Positive and Neg) from the battery. Now, some are fixated on the Fireplace. That is a 120 appliance and will not contribute to his problem. The Converter is connected and powers the 12 volt DC side of the RV. Just supplying 120 to it will not cause a malfunction. UNLESS there is 12 volt problems on the Output side. Since he is under warranty, he needs to STOP replacing parts and get the unit in to his dealer for a check out, to find the CAUSE of his problem. If he thinks he will get reimbursed for the converters, he is in for a rude shock. NO OEM is going to pay for self help that keeps destroying expensive parts. They may pay for one but not more than the one. Doug

The OP needs to clarify his first post. He stated "Breaker on Box tripped" Turned off and turned Fireplace OFF and turned breaker back on and Converter fried.
WHAT BREAKER ON WHAT BOX? The CG supply breaker to the 50 amp plug for his 50 amp cord? The breaker in HIS RV 120 breaker box?
If the CG breaker, then YES, odds are the CG breaker box/50 amp receptacle has a problem, like a open Neutral(probably BURNT) or shorted wiring which will indeed fry a 120 appliance, like the Converter, BUT, as I stated this scenario will also fry some of the other 120 appliances, not just the Converter