Not knowing how old your RV is kind of makes any 'guesstimation' difficult but I'll take a crack at it.
There's what's known as Field voltage (the F terminal on the alternator) that is controlled by wiring and circuitry sometimes going all the way up to the dash, then back to the chassis battery. Sometimes it's routed through an ammeter on the dash which tells you the charging status. If this voltage is low, it causes a higher voltage out of the alternator. This self regulating circuitry charges the starting battery quickly, then tapers off.
That wiring has a diode in series with the F terminal. If that diode is not doing it's job, you can get a higher voltage out of the alternator because the system is reading wrong and calling for higher voltage at the battery.
You can have other issues, like a corroded terminal or a poorly crimped wire. But my money would be on a bad diode. Usually these are located very close to the alternator, within a couple inches. They're inline, so covered with shrink tubing or vinyl tape. Checking all the terminals is important too.
Finally, you'd get more responses if you let us know the year of your RV and started your own thread rather then tack onto a thread from 2012.
PS I would not trust a Cummins shop that was measuring 24 Volt out of the alternator and just replaced it twice. They don't seem to know what they're doing.