Hi,
Do you have a converter someplace? Normally it will have a relay to disconnect the coach battery from the 12 volt appliances while plugged into 120 volt power. It sounds like this relay is not transferring back to the coach battery.
You stated that the engine and coach battery under the hood have power - 14.5 volts with the engine running. With the 120 volt unplugged, then the relay should 'drop out' and transfer the coach loads (lights ect) to the coach battery.
Most of the time there will be a fuse panel for the coach wiring. Behind that or perhaps in the same area is the 'converter'. It was a few years later that manufactures started putting in battery chargers, and stopped installing converters. (around 1992 for many of them, by 1997 for all the rest, because battery chargers are less expensive and more reliable than the older converters).
What you might want to do is bypass the old converter altogether. You can do this by connecting the input wire from the coach battery to the fuse panel. Then install a modern battery charger, and disconnect the power to the old converter. This will solve the problems of the converter 'charge circuit' from undercharging the battery with low input voltage, or overcharging when you have to much voltage, or are plugged in for more than a few days.
The 'problem' with bypassing the relay inside the converter and leaving the coach battery connected all the time is that a converter does not have a voltage regulator inside it. It will put out say 15 volts when you are connected to a 120 volt shore power, and perhaps 11 volts when connected to 105 volts input. So it might overcharge your coach battery, and boil out all the water, or might not charge it properly at all. You would have to plug in and measure the voltage at your fuses to know what it is putting out for sure. In my 1972 camper, the converter put out around 15 volts, and this will burn out the lights in a few weeks, while running 13 volts or less, my 1997 motorhome has most of it's original light bulbs, even though I have lived in it full time over 8 years!
Check out prices here for a new converter/battery charger. 45 amps is plenty for most uses.
Bestconverter.com Good luck,
Fred.
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Porsche or Country Coach!
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