Hi my name is ***** ***** me a moment just to check the diagrams and I'll come back to you.
OK and overview so you can understand how the thing works, there are no compressors ~ movement of the refrigerant is carried out by heating and the refrigerant circulating by convection. In your model there are two (three if you have DC supply) heating sources - 1 x 240 volt heating element (1 x 12 volt if fitted) and one gas flame. You have mentioned that the fridge works fine on gas so as you state there will be no problem with the refrigerant. You therefore have to check the electric heaters and firstly confirm that they have power going to them and if so if they are actually heating up. If you have no power going to them then the problem will be the main pcb. If power is there but no heat then the element has failed.
In the round I would expect the resistance of the 240 volt one to be around 20-30 ohms, not sure about the 12 volt one as we dont have any here and rarely change them. The major cause of this problem seems to be the main pcb and I note that not for the first time the latest pcb has changed. Usually an indication of there being a problem with a component. I'm afraod om the round Thetford have an awful reputation for reliability - perhaps a time to ponder replacement with an all electric model from on of the other manufacturers. I think that I have gone as far as I can - would you kindly rate the information given sofar, by selecting from the star rating system at the top of the page, once done all further questions are free. Thank you for using JA and have a good evening - Ian
There is no internal fuse shown Bryan - thanks for the rating.