Hi, this sounds as though you are having a problem with the ignition switch. If and when you turn the key off and back on, and it shows a full charge, do you have plenty of power like it is fully charged? If not, your battery's are in need of service, and they will not hold a full charge. The cells with in them will sulfate causing the positive and negitive plates to corrode up, and not hold a charge for very long at all, even though the gauge shows it is fully charged. If you have someone that can buy a simple battery cell checker at any auto store, this will tell you the condition of each cell on each battery. Very inexpensive also. This would be the first thing to check. If the battery cells show in the good to excellent range, you will need to have the ignition switch checked out by a company that works on them. I wish you the best on this, and I hope I have been of some help to you.
Sincerely,
Shastalaker7

I would recommed that you statr at the negative cable on the battery and remove it and cleant with wire brush or sand paper. Copper tube fitting brushes also work well for this and are available at most hardware stores. Leave the negative cable off and go through all electrial connections you can get to. Clean and replace one at a time. It is also a good idea to get some spray or brush on sealer from most auto supply stores to help keep connections clean and slow down on oxidation rate.
When you have done this then hook negative battery cable back up and see if you have made any improvement. I would also have battery tested to see what it is doing under a load.I would also use spray cleaner to clean out switch. Hope this helps you. Thanks