SO when you are plugged into power the power converter will take the incoming AC voltage and make the DC voltage and charge the battery. You will need a volt meter to see what the voltage is doing at the batteries, if the converter is working properly you should see around 13 volts and no lower then 11.8 volts when you are plugged in. Sometimes on the charge line from the converter you may have a breaker, below is a picture. As a general rule it would be 18" from the battery and can be on the frame. Most have a tiny tab on them that pops out when they trip, the voltage on each post should be close to chassis ground, if you have voltage on one side and low voltage on the other it is the problem.
The other issue is the converter, the converter will be part of the main fuse/breaker panel OR stand alone and plug into an outlet, the stand alone style will have two fuses on them, they can be located about anywhere, under the fridge, stove, behind the breaker panel or near the batteries. IF you can get me the make and model of the converter I should be able to send you some troubleshooting but you will need a volt meter.
As far as the cables if a cable is going to a breaker, like the picture below or a fuse it is going to be positive if it is going to the frame it is going to be negative. Normally red is positive white is ground, if you have two batteries it will depend if they are 6 or 12 volt on the main cables, below is a diagram for that as well..
IF you need more help please let me know, follow ups are included, if you are satisfied with my efforts please feel free to select a rating at the TOP of the page... Robert