2013 Chevy Equinox Battery Dying

[Islandsmith]

I've searched high and low. I've read a hundred threads. I'm lost.

Hoping someone can help me with a problem I’m having with my Chevrolet Equinox battery dying when I flat tow it. I have a charge line coming from the RV through a breaker and a diode wired to the positive terminal of the battery.

The lead from the RV is confirmed to be supplying 14V and I’ve confirmed this on the output side of the diode. If I measure across the battery negative terminal the the lead from the diode I’m getting 14V as well. However…as soon as I connect the lead from the diode to the battery the voltage drops to 12.6 measured across the battery.

I compared this to the car charging system. If the car is running (and not hooked up to the RV) I get 14V across the battery.

I’m at a total loss. Everything tells me the battery should be charging but as soon as the lead is connected something seems to be failing.

Anyone have an idea on how to trouble-shoot this issue?

[Islandsmith]

Adding a picture for reference.

A - the lead from the diode
B - battery negative

If the wire (A) is not connected to the battery there is 14V between A and B. If it is connected there is 12.6V.