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Trailer is a 2002. We've never used more than a 30 amp fuse and never had this problem with batteries charging
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30 amps was maybe big enuff for your old converter, but maybe not big enuff for you new higher output converter ?? Could this be why the fuse is blowing ?
At 11.8 no load voltage, your batteries are "dead", and the converter will be charging at maximum amperage. If your new converter is any larger than 30 amps, the fuse will blow. Most likely your new converter is 45 amp or larger. Replace the fuse w/ a 50 amp circuit breaker. The 8 ga wire may warm up just a bit for the first few minutes of charging, but it will be fine. If your charger is 60 amp, you mite be best to rewire from converter to batteries w/ 6ga.