I am late to this but I just read through the thread.
Voltage goes up when resistance goes up (V=IR). So you have a bad connection somewhere causing high resistance and voltage increase. From the testing that you have done that connection is on the house battery terminal of the isolator, probably the internal contact, but maybe worth cleaning the external connection as it is easy. Maybe check the resistance in the positive line to the house battery from the isolator. I suspect that if you replace the isolator and you will be good IMHO.