If you are saying that the battery supply to the inverter passes thru a master switch, I would suspect the switch...Leave dc loads connected to the switch (per former scheme) and connect inverter directly to battery positive terminal...The inverter needs to be connected directly to the battery with a 300a fuse in the positive cable.. Hopefully you’ve chosen cables of an adequate AWG size and a relatively short round trip cable run. It’s doubtful the switch could handle the kind of dc current the inverter will demand.
I’m using 0004 aught AWG for my 2,000w ProSine with a RT cable run of about 7’.
