So you are talking about ac voltage feeding it, if so that should be coming from a 30 amp breaker in the main breaker box.
If you are talking about dc, 12 volt, supplying it than it should be a straight shot of battery cable to the battery bank with a giant hundred amp fuse somewhere along the way. It will look like the old house fuses no clear area you have to use a meter to check. BTW if this fuse is blown you are not charging the batteries when plugged in either.
If you have both inputs fine then the transfer switch internal to the inverter maybe dead.
Ray, Cheryl, O'Connell and of course Miss Molly Silky Terrier mix
2006 Dolphin 5355