50 amp panel can be wired with one side direct power and the other side inverter only if that helps. Since you have the 30 it is fine also.
I would use an automatic transfer switch to connect the inverter. If no subpanel get a switch with converter outlet to avoid the loop. Not sure how the cam switch will have four feeds into the distribution panel that generally has one main connection.
I would go 80 amp PD converter with all that battery. Mount the converter close to the battery with #4 wire. No second connection to the 12v panel is needed.
Not sure about solar penney charge controller. Can it be programmed to automatically adjust between storage and in-use charging profiles? I think you may want more than 200 watts of solar so this might push into 24v panels and a quality MPPT controller.
I assume you meant 2 in series, 3 parallel for the batteries.