Thetford fridge not working on 12v dc

Hoping that there is an electrical expert out there?
My Thetford 300 series fridge (5 years old) works fine on 240ac and on gas but not on 12dc.
I have checked the supply to the heating element and it is 12.5v with the engine running at 1500rpm. And zero with the engine stopped. So I'm pretty sure that the split charge relay is closing. BUT why only 12.5V?
With the engine running the charge to the habitation batteries is the normal 13.1 to about 14.2V
I've checked the resistance of the heater element and there is continuity and 187K.

Any ideas please?

Thanks
Jeff

Thanks for that Peter.
I guess I was expecting that a failed element would be open circuit. Too much experience of my wife's oven!
I will now look at getting a replacement element. Available on e bay I think.
Incidentally I did find the schematic diagram after I posted and it seems that the fridge is served from a relay in the Sargent control centre and not as i assumed from a voltage sensitive relay in the alternator output. Would this account for the almost dead steady voltage of 12.5 at the fridge supply? Is there a voltage regulator in the Sargent?
Thanks for your help.
Jeff

Jeff -if you don’t mind me jumping on your post, can you tell me if these fridges are meant to run on 12v when parked (ie engine off) or only when travelling? Thankyou!

Hi Nangle. Definitely they only run on 12dc when the engine is running. As per Peter's reply the current draw would be far too high when parked. Habitation battery would fail within just a very few hours. If yours is the auto version it will automatically switch to gas as soon as the engine is stopped.

Jeff,

Like you, I’ve not known an element go into a high resistance state. What can happen though is a faulty connection (usually a plug and socket) that works loose and makes poor contact, creating a local hotspot and creating a high resistance path. I’m just wondering whether you disconnected the connections to the 12V element when you measured its resistance? If not, then your measurement could be influenced by another high resistance in the circuit.

The thread in the link below covers a known problem that can arise in the EM40 or EM50 (depending on van age) interface unit. The 12V high current supply from the vehicle alternator to the fridge 12V heating element passes through this unit.

You might want to check it out.

Tim 

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Hi Tim
Thanks for your reply. I didn't respond immediately as I wanted to go back and check all my measurements again first.
Yes I did disconnect the element before I tested the resistance but I tested it at the incoming plug below the control box. So the control was still in circuit! I've checked all the fuses(including the EM40 and the low current fuse [You must be registered and logged in to see this link.] at the EM500) and the high amp 12v supply and all is good. About 14v at the plug with the engine running. 
I've looked to disconnect the heater leads at the controller but the connectors are in the worst possible place hidden behind the wall of the van between the upper and lower vent grills. I'm reluctant to pull them off as I would then possibly have difficulty reconnecting without removing the fridge. Gas joint and all!

One last thing I can check is if the heater is taking a high current. I can do this at the EM40 fuse box. Job for tomorrow!
One more unknown: 
There are two 20 amp fuses at the EM40. Both labelled fridge. One is permanently live and the other switched when the engine runs. What is the permanent live for?

Anyway thanks for all the good advice. Interesting investigation but I am almost certain it is a faulty heater element and, if so, I think I will take it to a Thetford agent of just leave it. I'm not even sure how long it has been faulty since when one has switched off the engine and gone into the kitchen area the fridge has automatically gone onto gas anyway!  And presumably it has been running on gas when we are moving. Regards Jeff
Ps I whish that Thetford had included the fault tracing chart in the handbook!