Not sure what you are saying here. The alternator will put out 15+ volts if demanded by the sense circuit location. (my Clipper has a Delco 3 wire
If you are still using a battery isolator, chances are the alternator is being asked for full production at all times, as there is no physical method of controlling the demand of the house and chassis battery demands.
I have upgraded my systems to use SurePower 1315-200 battery separators controlled with solenoids, to allow manual override of demand on the chassis alternator.
Are you having battery charging problems? If not, it may just be the gauge or a poor ground at the gauge???