12v Caravan circuitry

[LaverdaDave]

I have a portable fridge in the caravan. It errors when connected to the fridge circuit in the van but does not error if I connect it directly to the andersson plug input to the van.
The fridge has L/M/H cutout circuit and if the voltage gets low the fridge is designed to cut out and error.
If the fridge is connected directly to the batteries via the andersson input of the van it will run for days without error but if I connect it to the fridge circuit as soon as the fridge starts to draw higher amps the fridge perceives the voltage has dropped too low and the fridge errors. I had an auto-electrician look at it but he could not find any fault.
What circuitry is likely to cause the fridge to perceive the voltage as low when the amperage draw goes high?
I also have a electrical circuit meter that I can place between source and load which does show a minimal drop in voltage as the amperage goes up. E.g. Batteries show 12.6v when the fridge is connected. As the fridge draw increases the voltage declines to about 12.1. (this is when the fridge is connected directly to the andersson input.
But if the fridge is connected to the fridge circuit the batteries show 12.6v when the fridge is connected but as the draw goes up the voltage drops to under 10.5v and the fridge errors.

 

[littlespark]

Faulty battery at guess.

I would expect to read 14v from a fully charged leisure battery.

 

[Lucien Nunes]

But if the fridge is connected to the fridge circuit the batteries show 12.6v when the fridge is connected but as the draw goes up the voltage drops to under 10.5v and the fridge errors.

It sounds like the resistance of the fridge circuit itself is too high. When the supply is taken direct from the Anderson connector, the voltage drops from 12.6 to 12.1 i.e. 0.5V drop at full load. When connected via the fridge circuit it drops from 12.6 to below 10.5V i.e. more than 2.1V drop at full load, which is too much.

Likely causes are either a bad connection at a junction box / splice / inline switch etc somewhere along the run of cable, or the cable itself is undersized for the load.

What size is the cable? How long is the run (approximately?) and what is the fridge model?