First, let me just say.. winter please depart... Second I think the pressure regulator on my shore water is old and in need of maintenance. The flow at the dock is great and inside the sinks start out well but then the pressure drops off. I think this unit needs to come out and at least be cleaned or replaced.
Any thoughts here community?
Clean the aerator on each faucet.
Do you have a gage downstream, if not can you rig one? It's good to have information, a rap with a hammer, a turn of the regulator screw may solve the problem.
I also have a 1980 Commander 410. While mine is still very much in the middle (beginning?) of restoration, I did at least get the plumbing working. (Plumbing was something I could work on in parallel while waiting for little if any engine progress...)
At this point, I don't have the shore water hooked up at all. The regulator and valves were old and suspect. The valves are old gate valves that are probably long since either stuck or would leak... And no idea what the status of the regulator is.
If I ever go for a shore water connection, I'll replace the entire works. Though it's still not quite clear to me if it was intended to be able to fill the water tank from the shore water connection.
Part of my decision is biased of past bad experiences with high dock water pressure blowing out water lines. I have a 100 gallon water tank (at least according to specs...). That seems like PLENTY. I'd much rather fill the tank and run off the water pressure pump. In the absolute worst case of walking away from the boat with the water pump still on - at least I'd be limited to no more than 100 gallons of water in the bilge when I got back (and hopefully some amount of that already pumped out by the 4 bilge pumps!).
I would certainly NEVER leave shore water connected when I was away.
As for regulators... I've used the "RV" style water pressure regulators that screw inline to the hose. They seem to be reliable, and would be trivial to replace from time to time if they ever started clogging up. The valve and regulator matrix that originally (?) came with the boat will be extremely challenging to remove / repair / replace.
Replace it with one that is of all bronze construction. I went through the same thing when the original one with steel innards went bad on me during a holiday weekend. The bronze version was an instant cure to the problem.