Any gauge can malfunction, even the most reliable ones.
It sounds like something is wrong between the tank and the indoor gauge; an open circuit, a short circuit, a stuck sensor, etc.
You can take it to a qualified service person and have it checked and repaired, or you can just rely on the tank mounted gauge (if it is really working).
Personally, I would want the dang thing to work the way it should.
The repair might be simple and inexpensive, or it could be rather pricey.
good luck.
CM1, USN (RET)
2017 Jayco TT
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