Roger,
Along the lines of what Steve has brought up, any loose or corroded lead in that path of the alternator sensing voltage can cause the regulated voltage to be unstable. Along with the sense wire I would check the battery post leads, the + sense lead that Steve brought up as well as the ground wire on the alternator and where it ties to a post or RV ground engine/frame. Any dropping of voltage on the sense + to ground (which can also be caused by the ground side), due to real load changes or bad connections, causes the regulator to signal for increased output. If the connections are all good and the problem persists then the alternator should be checked out.
From experience, I had this issue when one of the smaller wires to the regulator was down to just one or two copper strands due to vibration breaking strands at the crimp connection of the lug connector. Hard to see, found it wiggling the wire at the connector. A good practice is to form a coil of a few turns, like a spring, right before the terminal to dampen any vibration at the lug if you find you have that issue.
Lastly with the fridge is pulling a significant load through the inverter and then having the fridge compressor kick off makes me wonder if that change in load is somehow causing an alternator response that produces over voltage. It may be worth seeing if the problem voltage change can be toggled on and off with the inverter on and fridge compressor being on and then the load being off at moderate engine RPMs.
Being cautious, if you see the problem again and can get some voltage meter leads from the sense wire to ground and that voltage is stable while the alternator output voltage is wandering then it is the alternator regulator or or internal components.
Later Ed
