Karl,
Intermittent problems are tough to figure out and I can see why this is driving you crazy.
The comment from Paul about the starter is a good one. On cars, the battery lead can go right to the starter and that lug can also be used as a terminal point for the alternator output. For our case I belive the alternator output goes directly to the center terminal of the isolator only. There may be a ground lug to the starter frame involved. Good to check them all and be sure they are tight, clean, and if very exposed to water, perhaps it would be worth a try to apply some liquid tape there also.
Usually the ground lead from the alternator is terminated close by on the top region of the engine. Look for a single wire or a small bundle of wires to a ground lug. Given this problem only occurs under the most severe conditions, maybe water is getting into the alternator brushes or internal connections until airflow and spinning forces intermittently clears them. I have not heard of this, and given it would be an uncontrolled current path situation, I would expect some components within the alternator to fail if this was happening. Things like the diode pack or regulator.
You could also have good clean corrosion free connections in all cases and rather than a series voltage drop in one of the normal current paths you could have a parallel current path (short) created by the water connecting a plus lug to a nearby metal ground. This load, like a short or near short pulls the alternator output voltage down until enough of the water is cleared and the short is removed.
I'm not sure if this is possible but there are some clamp on DC voltmeters today. I think I saw one at Sears for around $60. Seems like if you could put the clamp on the alternator ouput wire and tape it/tie wrap it safely in place. Run the wire leads into the coach and set up the meter in the closet or bedroom to get some diagnostics while the problem is occuring.
What would be good to know is if there is a very significant current flow that coincides with the low voltage readings that you are seeing, or if the voltage drops without a high current output. The high current out would indicate that all is well with the belt drive, the alternator internal components under wet conditions, and that an external short is drawing current. If the voltage drop occurs without a high current output I would look at the ground lead or the possibility that water is intermittently causing regulator problems within the alternator. If it is the high current case you could then begin to move the voltmeter clamp to various positive wire locations over time to see where the current flow is going.
Tough problem to sort out.....hope this helps.
Later Ed