If you are trying to avoid having a dead
engine battery from running the fridge on 12v then I think Roadtrek already built that in with the isolator in your '94. My '04 had it.
The fridge should be connected to the house battery. When the engine is running, the house battery and the engine battery are combined so that both get charged. When the engine is not running, the engine battery is "isolated" from the house battery by the isolator. The fridge could run the house battery down but not the engine battery.
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If you are trying to avoid having a dead house battery from running the fridge on 12v that's more involved. In my RT, I just had to remember to either switch the fridge to electric, off or propane, depending on what was best at the time to avoid running the house battery down.
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Dave posted great info about Battery Cutoff Devices (retail and home-made) on his site here:
http://www.roadtreker.com.....Battery Cutoff Device
Here's the link to the rest of Dave's site:
http://www.roadtreker.com/