Battery Issues

We have about 18,000 miles on our TrailManor 3023. We very quickly replaced the single 12 volt battery delivered with the unit with two deep cycle 6 volt batteries. These have worked very well for many thousands of miles.

However, the trailer batteries never receive a full charge while being towed. I can think of only one explanation for this which comes from my boating days - battery isolators. On boats when there are several battery banks (house batteries & engine starting battery), to avoid one system discharging the other, diode isolators are installed. This works fine providing that there are diodes on both battery banks and the alternator is adjusted properly. If there is a diode on only one battery bank, the other battery will never get a full charge because there is a one volt plus voltage drop across the diode. I suspect that this is what is happening in our trailer.

Has anyone else had this problem? And if so, what have you done about it? Does anyone know where the isolation diode is located?

Thanks