Billy,
A Battery Minder is the way to keep engine battery charged by shore power because shore power does not charge it through the converter. I asked the manufacturer if it was ok to leave the Battery Minder attached to the engine battery at all times and they said yes.
At seminar given by one of the developers he said to keep in mind that if there is no 120v power, the Battery Minder connected to a battery draws a small current to run the high frequence oscillator that does the desulfation.
I leave mine connected unless there will be a period of storage without shore power.
Dick in FL