Here is a datasheet for the R3D
http://www.rochestergauges.com/products/R3D.html
the sheet states;
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Operational Voltage Range
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3.5 to 6.0 vdc/ratiometric.
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Output Voltage
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Ratiometric 5-80% of input voltage @ 5-80% volume.
There seem to be only three connections; power in, ground, output.
I guess if the 20' cable has low resistance the meter could be read but somebody with electronics knowledge needs to confirm that.
The meter is only accurate to +/- 4% of full scale and that excludes float errors which could well be the major error. I think the guage will give a good indication if the tank needs filled but it will not give an accurate reading of the volume of propane delivered.