We have had to replace the 240 element on our thetford fridge twice in the last 3 years, . Access for checking is through the lower external fridge vent from outside the van. There are two plastic sliders at the top of the vent which release it. Inside fixed to the back of the fridge is the electrical control box . Ensuring the van is not connected to the mains , or has an
inverter, the cover of this box needs to be removed by releasing the cover screws which are quite well hidden , there are 3 I think. There are leads out to the elements which are located in the flue above the gas burner. The leads to the elements run through a blue protective sleeve which is printed with a part number and the voltage of the element. This will enable you to tell which is the 240 one. As Mr Brown says measure the resistance across the terminals of the element. Both times my elements showed no resistance and had failed . The parameters are actually to be found elsewhere on the forum so if you get a reading let us know. If you get a zero reading the the element needs replacing. I struggled for ages at first but was unable to change element without removing the fridge. The fridge is secured with screws hidden behind little plastic plugs inside the fridge. It is necessary to disconnect electrical connections at the rear . Take a photo with a digital camera so you can recconnect in the same order. Also the gas needs to be disconnected . Turn off gas at bottle . I found the easiest place on my van to be at the elbow where the gas line comes through the base f the cupboard. Before starting to remove fridge get the new element . Once the fridge is free you will probably need another pair of hands as the rear of the fridge is quite fragile and space will be very tight. When you get the fridge out you will need to cut cable ties holding the element wiring . Take photo to show route of cable. I found it easier to remove the flue shield which is accomplished by squeezing to release the retaining tags. The element can the be slid out of its housing. Reassemble with the new element. Run cable and tie.
Replace fridge into its housing. Redo electrical connections, replace cover. Redo gas connection hand tight then nip it up plus eighth of a turn. Get a plant water spray with some fairy liquid in water solution keep it handy. Take a moment , all connections made, all screws back in fridge secured, everything looks right nothing caught. Ok power up 12 volt supply, check fridge display, power up 240 , check operation, . Right turn everything off . Turn gas bottle on. Spray around remade connection with soapy water. If it bubbles give another eighth of a turn . If I need to tell you to support the joint so you don't strain any other parts of the gas line you should not be doing it. If all ok turn on electrical supply and run fridge on gas. It may take a couple of goes to purge any air in the line. Once lit give it a moment to run then turn it off and recheck with soapy water. All should be ok , if it's leaking and you need telling what to do get a gas engineer. If it's all ok replace van side vent put tools away and have a beer . Oh by the way the screws on the electrical cover are star drive heads.
ONLY DO THIS IF YOU ARE CONFIDENT OF YOUR ABILITIES