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It should switch over to battery automatically. The charging side of the converter may not be working OR if the battery disconnect switch is in the wrong position that can do this as well. If you have a small digital mutimeter this is pretty easy to test for. About $20 or so at most hardware stores and easy to use. I can direct you in some testing. All follow ups are included in our fee.
Some are mounted on the dash.......usually on the left hand side. Some are mounted by the entry door. Rocker style switch. Some would say "Aux" or Battery Aux" on them. When you hit them you should hear a distinct "click" as the solenoid closes.
I don't think the isolator is the problem. That just allows the batteries in the house side to be charged when the engine is running. This format depends on multiple exchanges of information and sometimes it takes a few back and forth discussions to arrive at a solution. Rarely, is it resolved on the first one. There is only one fee no matter how many it takes. The only "extra" fee is if you want to do this over the phone. I don't charge my customers any more for multiple calls. If you have to go to a shop for repair or call in a mobile tech, you will see that our fees are a great deal! In any event, you can always request another tech here or ask for a refund. Either one will be granted quickly. Hope that explains this process a little better. Let me know if you want to continue or he needs help with the testing.