Digital volt meters can be misleading under some circumstances. The voltage is sampled for a brief period about 4 times per second so that if there are pulses or fluctuations on the line it can occasionally be reading at the time of a peak and again at a low point. This can be checked or overcome by using a resistor between the meter probe and the supply and fitting a largish capacitor across the meter. A 10K ohm resistor will reduce the reading by .1% so can be ignored and a 1 to 10 microfarrad capacitor will act like a mini battery to ensure the measurement is an average. Wait a few seconds for the reading to settle.
However back to the posters concern.... If the voltage is measured at the actual battery terminal it should be fairly smooth and accurate. Of course the charge voltage should be close to 28volts.
If it really is fluctuating that much then check the brushes and skip rings of the alternator these may be worn with low brush pressure resulting in dirty / burned slip rings and so unreliable contact. good luck olewill
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