Shorepower issues

One 1 cord, you'd have shorepower 1 on, the main on the left side on, and you have a choice on the right side, if you put it on inverter (also a charger) it will buffer startup loads a bit and you might not trip the shorepower. Looks like you should have shorepower cord 1 plugged in, if only using 1 shorecord.

L1+L2 current need to be kept under 50 amps on one cord. At the dock I'd run 1 chiller in the winter on Neptunus'es and it would keep up no problem in South Florida. Summer time you could run off of one chiller but when it's mid-day and 92F outside 1 chiller has trouble keeping up so interior temps might creep up 1-2F during peak heat but can be done. If you haven't had your chillers cleaned recently, have them cleaned as when they get dirty they use more amperage.

Does your shorecord have the factory glenndenning plug on it where it plugs into land? You can tell because the rubber boot it is clearly 1 molded assembly to the cord?

Or jump onto a neighboring vessel with a good ground/bond system to shore power

At the Source, on shore, at the main service, the neutral ( grounded conductor )and ground ( grounding conductor )are bonded together. They are common only at this location if wired correctly. ( not in the pedestal or any sub panels )
There is a driven ground rod at this point.
It can be attached to the transformer secondary or in the main service panel, both are acceptable, but only one location.
And only one location that the ground & neutral are terminated together, at the main.
Sometimes the water is a better return path for fault current than the system ground.
It may try to return through the water/earth and to the driven ground back to the shore power transformer source.

The shore power cord for an isolation transformer is the A-phase and B-phase and a ground.
The ground is be bonded to the isolation core.
This protects against a ground fault up to the isolation transformer only and would not be tied into the boat ground/bond system.
If the A or B phase fails within the turns of the transformer, it will fault to the core, or phase to phase & the fault current will travel back up the shore power cord, through the system towards it's source, building current and tripping the pedestal breaker, ideally.
The neutral from shore power is unnecessary because you are creating a New System which creates it own neutral.

The secondary side of the isolation transformer creates it's own, new system.
The neutral and ground are bonded at the transformer or the main panel only.
They are not common in any way with the shore power ground.

The bonding system and neutral of the boat are common only with the new system.
isolated from the shore system.
If there is a GROUND FAULT the over current travels the ground/bonding system back to the source, the isolation transformer.
The ground fault current on an isolated system has no reference to the shore power source and ignores it.

If the ground conductors are open somewhere or heavily corroded, the ground fault current will attempt to get back to the source any way possible. which MAY mean it travels through the sea water from one fitting to another.

Heavily corroded ground/bond wires also slow the reaction time of the over current device, usually a breaker.
Slower breaker time can cause the current to really build and start letting the smoke out of appliances or cables.

also when people start mixing neutrals and grounds out in the system, then the return current from your appliance will not stay on the insulated neutral, but some of it will travel on the ground/bond system.
This is bad.

The ground system should only have current on it in the event of a fault. The bonding portion of the system collects minor ( but can be detrimental ) stray currents on the system.
The bonding side of the system is supposed to make everything attached to it the same potential.
All equal.

This so the engine block cannot hold a 120 volt charge from a faulty block heater and knock you silly when you touch the block and, say, a copper water line. All of the metal, non current carrying parts are the same potential and bonded back to the new source.

The stray or goofy currents from poorly wired neighboring boats will ignore your system because it has nothing in common with the neighbors system.

OK
Had a marine electrician on boat today.
Everything should work with one 50 amp cord if not drawing more than 50 amps.
We shut off all power draws on the vessel and asked the one cord to handle one chiller and one zone AC.
We put on the air handler and when the compressor was turned on it tripped the circuit. Neptunus and my electrician think it could be a power surge at start-up and are suggesting a “ soft start” unit be installed.
My question is if I use a “Y” adaptor to connect my two -3 wire 50 amp power cords to a single 50 amp shore inlet would that work?
I am trying to make my vessel fit into more marinas that might only offer one 50 amp outlet.

PS. The boat has an isolation transformer on line one.

The shore end is not factory

If the one chiller will run on the generator it should run on 50 amp shore power. Yes, switch cords....

Should have said good 50 amp shore power, all is good on the gen. You mentioned a list of items that should be checked before adding $oft $tarts.

Update
Tried using only line 1 with the third chiller only on (the one most outboard) and it worked.
Don't know why?

Is the third chiller newer? I am guessing so, since Neptunus only installed 2 chillers from the factory on these boats. A/C compressors will usually pull more amps at start up as they age. Soft starts will definitely help.

I believe all 3 chillers are new and were replaced together.

Home ac tech switched to larger start caps on home ac's for backup gen to handle ac start up loads, it worked.