[Motor31]

What Huck said. What kind of battery bank do you have, dual 12 volts or dual 6 volts?

Other consideration before you hook up the charger is this. Check for voltage at the battery as it is now hooked up. Push the disconnect switch in and check voltage after it sits for a few minutes using a digital volt meter if possible. If there is a voltage drop the converter is already charging the batteries and adding another charger won't do anything for you. If the voltage is still dropping after they sit a few minutes with the switch in one or both batteries may be bad.

Disconnect the negative cable from the trailer and the negative cable connecting both batteries together and check voltage between them. That will show which one is bad. The bad one should have significantly lower voltage if it's a bad cell.

If one is bad and you have two 12 volt batteries you can just cut the bad one out of the system and keep going a while if you want. If you have two 6 volt batteries you will need to replace them both. Just swapping one at a time will end up wearing out the new one quicker.