When full charge is approached, a diode isolator still drops from 0.55 to 0.7 Volts. This voltage drop reduces the voltage available to the battery which inhibits full and complete charging within typical operating times. When you install a second or more batteries either in your vehicle or towed vehicle, you need a way to prevent discharge of one battery circuit as a result of a load on the other. The least expensive method is simply to disconnect your trailer plug every time you turn OFF your engine. It doesn't work if you don't disconnect.
Some connections are frustratingly difficult to remove.
If the spare battery is in your vehicle, the choice of disconnecting and reconnecting is even less desirable. To make the isolation more automatic, some people will place a diode in series with the alternator and main battery. As a result full charge will not be achieved unless you operate the vehicle for very long periods. Often, the vehicle is not operated long enough and then, the battery sits for a while with less than full charge. This condition causes the battery plates to sulfate, which will gradually degrade its performance and lead to early failure.