House power
1) measure house battery voltage >12V good enough ,11 replace batteries
2) search back from battery for voltage to 12V to power panel; 50A breaker and main power switch ("use" switch) .
The engine alternator should have been charging the house batteries if the switch was in the use position and the power relay is functional. I assume, in your case , the house batteries are too low to switch this relay or you have blown the 50 A protection fuse (located close to the batteries), maybe due to a house battery shorted cell?
This should be easy to locate if you are methodical is your analysis. Good Luck
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