94 Southwind House Batteries. Begging for help :(

both batteries are 6 volts and are measuring the proper voltage when disconnected.

both batteries are brand new

when measuring voltage I have

6volts on each battery
6 volts pos to pos
6 volts neg to neg
0 voltage from outside pos to outside neg

circuit breakers blew when I put batteries in backwards. I reset them.

fuses tested

both disconnect switches click

cannot find any fuses on the converter.

what's the proper way to test for voltage on the disconnect coming from the batteries?