Welcome Sue Store.
I echo the Prof’s advice. Gas is dangerous so please be warned.
Facts. You say none of the three burners will stay alight?
Is there gas in your gas bottle?
Before you ignite the rings can you hear/smell gas coming through?
When the ring ignited how long do you hold the gas tap depressed?
Gaining access to the thermocouples is not an easy job. Other gas appliances , oven etc , may need to be removed.
You can remove the tops of each burner , two small screws. Lots of rust collects so cleaning out with a vacuum cleaner may help. Otherwise get a qualified mobile caravan engineer to sort it for you. Sadly people do die in caravans from Carbon Monoxide .