I recommend that you use a voltage meter to determine voltage reading. The tank gauges operate on 12v. Should have nothing to do with whether the 120v shore power is plugged in or not unless you don't have a battery installed.
If the voltage source is related to shore power, you likely have a ground issue which could be serious.
Using your volt meter, put negative in the ground under the rv and place positive on various steel frame components under the rig and see if you get same voltage.
If you do, then you need to look further for the source.
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