Weird power issue

We moved to a new slip at our yacht club, boats have started coming out and we are consolidating a bit at the very end of our season up here.

Anyhow, so, I have 2 x 120v30A shore cords, on separate panels. One is air conditioning the other is main ship’s service.

Since moving to this slip, a load on the AC panel RAISES the voltage on the main panel.

So, main panel is 120v with a 2000w load on it.

Adding a 1500w load to the AC panel raises the voltage on the main panel to 135v.

I have tried different outlets on the same
feed, same issues.

What is happening here?

The dock power is 240v, with multiple 120v outlets per box, so I have tried some combinations.

No transformers on my 43DC.

I will say, this is a known problem on this box that everyone thought was fixed.

But, no one knew what caused it and clearly it is not fixed.

So, it is not an issue with the boat.

I unplugged the AC side cord and I can move down the dock 20ft or so and reach another box that is fine.

But what would cause this?

Our club electrician has been MIA, but I discussed the issue with a local marina owner who is an electrician, and he says it is probably a floating neutral that shows itself under load.

I'll be moving to a different box!