That is a big question, there is likely to be a seperate gas tap but where it is depends on the fire and the van. Do you have more information and a picture?Gas fire not working is the a separate tap like there is for the cooker?
If so where can I find it please.
On my 1995 m/h the three are Hob, Oven and Fridge.There’s 1tap in bottle locker. And think 3 by sink that does the cooker
Quite confident working on things.If the heater has not been used for a long time it will take a long time to get the gas through. If you are not confident working with has appliances I would advise gett a professional to have a look