In the case of our 1996 Jayco Designer 3030RKSS, Jayco used a Todd Engineering converter (perhaps you know this, but in case you don't, the converter converts 120 VAC shore power to 12 VDC to power the 12 VDC loads in the 5th wheel - including the lights - and to charge the battery(ies).) These Todd converters were prone to failure, and ours quit on a cold, late October night camping near Gatlinburg, TN. We didn't know it until the furnace quit in the middle of the night. It's easy enough to check the converter output with a simple volt-ohm meter.
I don't know what converter Jayco was using in 1990, but if all the breakers are turned on and the converter is plugged into 120 VAC, it may well be that the converter has failed. If you find that this is the case, I would strongly recommend replacing it with a modern multi-stage output converter such as the Progressive Dynamics 9200 series. This will keep your batteries from boiling dry when you're connected to shore power for long periods of time.
Yes, you can use an automotive 12 volt battery charger to charge your coach battery(ies), but be sure that the batteries aren't dry before you try to recharge them.
If you can find an experienced RVer with basic tools, or one of the park's maintenance personnel who is willing to help, it's a pretty simple troubleshooting procedure to determine if the converter is working or not.
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