Heater issue Voltage drop

[BelfastVan]

So I have a Transit The starter and default house nattery (A transit comes with 2) are under the driver seat and
2 Halfords Leisure Batt-HLB700 - 100Ah under the passenger seat.

A Renogy 12V 50 DC to DC controller behind the driver seat (No solar connected yet

Thick leads going up to the controller and to the passenger side house batteries

I have a second "patch" board behind the driver side as I like to get my left side wiring from there and my shore / AC to DC charger positioned here
I am measuring 13 V at points A and B (see image)

Point D drops 0.5V ...and at the point my fridge kicks in the voltage drops and goes back up when it goes in "standby"
Point C is the bloody issue though

 

[wildebus]

Nothing unusual in what is happening.
The usual reasons are inadequate and overlong cables to the heaters and the fridge.

Try to get rid of excess cable to the heater (if this is one of the "Chinese Diesel Heaters", they come with a very long loom and many people instead of removing the excess, they just leave it in place). If the heater is a fair way from the power source, then upgrade the cable gauge (this is not about sufficient to carry the current but about minimising the volt drop).
What you can also do is fit a Voltage Booster close to the heater. Get one that is rated around 200W which will be fine for the Heater when it needs to activate the Glowplug.