I did a little more thinking about the problem you pose:
1. The post about being sure the batteries are isolated~the '95 EVC has a solenoid that connects the deep cycle battery to the charging circuit when the ignition is on and disconnects it when the ignition is off.
2. If the battery loses its charge within a couple days, it may be the battery is the problem. When I bought my EVC it had been outside on a dealer's lot for the winter. The propane alarm had discharged the battery. A discharged battery is much more susceptible to freezing than a fully charged one. That had been a cold winter and the discharged deep cycle battery had frozen cracking the case. There was just enough electrolyte left in the battery to make it appear that it was working, but it didn't last long. I had to replace the battery. The life of a deep cycle battery is much shorter than a automotive battery anyway. So---Check the electrolyte level in your deep cycle battery. If any of the cells have plates that aren't covered by electrolyte, remove the battery after disconnecting the leads, and check the battery case. If it's cracked, the battery needs to be replaced.
3. Note that if you connect an external charger to the deep cycle battery, it should be disconnected from the vehicle first. I use the converter to charge mine. Simply plug in the electric cord and the converter should charge the deep cycle battery if it's working right.

Good luck. Let me know how it works out.

Marve

'95 EVC

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