Dang.. Do not know what model Magnatek you have but recommwend upgradeing
The rest of this is specific to the 6330 but may cover other models.
The 6330 is not like modern converters it is a converter that outputs raw unfiltered rectifiesd A/C regulated by a ferroresonate transformer.. NOT GOOD AT ALL, and a small low current charger thats also poorly regulated A relay bridges the two if no shore power. Kind of like this
Battery---30 amp---T1---Relay---T2--30 amp--branch circuits.
T-1 is the charger and a couple of branch circuits (IE: Radio) that truly need clean DC.. T2 is the converter
so to run on batteries you need BOTH fuses. but one and I do not know wich will do on shore power.
IF one is blown. the other likely is as well but check it. replace BOTH with power off (Battery disconnected shore unpluged) or better yet upgrade the electroincs with either a progressive 4600 or 9200 (46 for "Converter is part of fuse panel) 9200 if it's a stand alone.. Your batteries will thank you.