Update to my own message above:
I've been running for just over two days like this - DC Regulator Fuse removed (F1 on the schematic), and battery charger (a Guest 12v 2 Bank
https://www.batterystuff.com/battery-chargers/12-volt/marine-chargers/GU2611A.html). This is plugged into the convenience outlet, and appears to be working (it is normally used to keep the batteries topped off when not running) - but I now have dead batteries.
As commented above, I noticed that the panel volt meter seemed to be be reading the battery voltage when I started the unit with the fuse removed, and was originally high in the green. Over the last two days, the voltage has been dropping steadily, and now is just in the red. Initially the battery charger LEDs indicated charging, then charged (everything looked good), and then moved into some kind of error state (both red & green LED's flashing) this evening. Batteries are pretty much dead - they won't crank the unit over after I stopped it this evening, and only TOTAL about 15v when the unit is running and charger disconnected.
I have just managed to jump start it, and it's running well (even with the jumping batteries removed) - but with the DC volt meter in the red, very slightly higher with charger attached. I'm very concerned that it's going to die - I'm assuming that at some point there won't be enough current to hold the stop solenoid in. I assume that it, and the aux fuel pump are the main consumers of DC current when the unit is running (?)
I'm very surprised this is happening - the charger is connected, and I think it's good. I am wondering if it somehow 'confused' in terms of battery conditioning because there is current being drawn - or even if the blown regulator has some effect, even with the fuse F1 removed (looking at the schematic it's not clear what the fuse does - both ends are connected to the regulator). It might also be that the short period of unregulated 33v somehow cooked the batteries. However, I can't really investigate this while I need the unit running.
Looks like we may not get power back for four more days (current prediction). I may buy two batteries as insurance tomorrow, and keep them charged (externally) to swap in. The originals are about 8 years old, though they seemed good - but I don't think I can run the unit without batteries AND the F1 fuse removed... (new regulator on the way, but probably 4 or 5 days out...about when we're scheduled to get power back....)
Apologies for the long description - I wonder, has anyone seen anything similar?
-Steve