Was the key turned on during fuse testing? Was testing done by touching tip of tester to tiny hole in end of fuses? Sometimes the tester tip has to be very sharp to make a connection in the tiny hole. I have been fooled by this in the past.
If your indicator is "on steady" with key on, engine off, then a volt guage should act the same way (with resistor removed or bypassed).
If you wired the guage into the circuit BEFORE the resistor, you could have both guage and indicator, I think.
The guage would be active whenever your relay was energized (the relay you set up to send power to the isolator). Describe for me how that relay acts if you turn the key on, but the engine is not started.
Yes, I think that would work, but the engine would have to be running for the guage to work. If you want to know "How much voltage is left in my frame battery after running my room lights for an hour?"...wiring to the dash indicator wire would be the wrong way to go.
Maybe I am asking this question in the wrong way, but...what do you want the guage to tell you? If you want to know "the condition" of the frame battery, a volt meter would be a better choice if you want to know the "state of charge"....(how many volts in the battery).
To easily monitor frame...
That type of amp guage is typically installed inline between battery and alternator. Your charging system may be too "large" for that ammeter to handle. But, you have an unusual setup and so I must ask...what are you trying to monitor?