The electric in my caravan trips whenever the fan heater is Not running, as soon as I turn the heater off a minute later the fuse trips, it also stays on when the kettle is boiling but when the kettle finishes if the heater is not also running the fuse trips a few seconds later.

dethleff,
Can you provide more information here?
Is this a car? a trailer?
How many things are you running at one time?
Do you know the voltage and the current of each device?
What size is the fuse?
The more you tell us, the better the answer...