[77newell]

Yachts: I'm going to assume the voltage that is dropping as you drive is the chassis voltage and not the house battery voltage. When you turn on the generator and merge the voltage rises to the normal level. This says to me that the gauge and wiring to the gauge is probably working fine. The battery terminals are probably OK since both the gen/merge returns things to normal and you did not report any hard/slow starter which I would have expected would be more noticeable at high starter amp draw than with just the alternator pushing current.

Assuming (yea, maybe not the greatest idea but we have to start somewhere) the alternator is fully functional, then the current is getting cut off somewhere between the alternator and the batteries. You may be able to further confirm this by observing the battery charge rate which I would expect is negative - a discharge (which is a dissed charge). If all this is as I have suggested, then I would start measuring voltage at the alternator with the engine running - beware the moving parts - and follow on down the wire to the battery until I found the lower voltage at which point I know what is broken. You want to measure at the bolts sticking through the wire terminals rather than the terminal itself to make sure the electricity is getting out of the wire and terminal to the intended destination because as others said above it is not uncommon for the terminal to be the problem.

Please let us know what you find so the rest of us can more easily solve our problems should a similar situation occur.