First on a rig that old you MIGHT have a progressive dynamics (I have had PDI hardware that old) but odds are you have a Magnetek 6300 series. How to tell: Open the door to the fuse/breaker panel. Look on the inside of said door.
This converter is not what I'd call worth much.. An upgrade is indicated.
now the Progressive Dynamics IntellaPower 9200... There is no upgrade.
Second, since you pulled the batteries from your boat, this means they are Marine/Deep cycle. and since you took 'em down to 9 volts.. Wave By-By to em..
IF you have the headroom, Replace with a pair of GC-2.. More amp hours, less cost and true DEEP CYCLE. those do not like 9 volts either. but Mine have recovered from that deep a few times (They finally died, 9 years after they were installed I get new ones in 1-2 weeks when I get paid and move the house to the battery store).
Home is where I park it!