It can’t be your alternator or your house batteries would not charge also. I had the same problem it was the battery isolator The blue box $102 very easy to change you said you checked it I would take it off and have it checked at a alternator shop
89FE3AD2-260B-49E8-8928-53E4A8469C5B.MOV
89FE3AD2-260B-49E8-8928-53E4A8469C5B.MOV