Thanks for the added info as it helps to see what might be happening! But it also brings some questions about the spike.
I define a spike as a sudden increase in the voltage to past some limit which makes the frig shut down to avoid damage. that is often something like a lightning strike or really strong.
But the batteries can't make a spike. They are just storage for what the rest of the systems produce. the alternator is what produces the voltage when it is running but that is where things get awkward as engine alternators on cars and trucks are not famous for spikes as much as they are for having a slipping fan belt that makes them turn slower and give lower voltage than we need!
I would not expect it to be the solenoid as it only operates to connect the coach battery and the chassis batteries together and can't make any higher voltage than what the batteries have in them plus the alternator. So if you have a battery at 12.5 and one at 13 plus the alternator running as fast as it can, the top voltage should never be over 13.5- 14 volts! But the frig should never shut down on those levels as it is normal for the alternator to put out that higher voltage until it gets all the batteries charged and then it cuts back slowly to avoid overcharging them to boil them dry!
This is where things begin to get even worse?
The manual for your frig is here:
https://norcold.com/wp-content/uploads/2019/02/Owners-Manual-Through-02-21-06-n62.pdf
Dropping down to page 17 to look at the error codes show this:
View attachment 187212
Sorry but it seems that is as far as we can go and I might suggest calling Norcold for advise before trusting the repair folks too far???