Start by inspecting the heating element for shorts to ground and for damaged or melted spots in the heater its self.
The 2 most common reasons for that fuse to open are the heater and the pump, just to make sure We are thinking of the same fuse You said in Your post the one in the control panel, the one snapped onto the side of the control mounting bracket?
Why dont You go ahead and send Me a model number and I can make be sure I am thinking of the same model.
Even though You dont have heated wash on, You may have heated dry on which uses the same heater, also the heater will come on if the water temp is to low.
Try to physicaly check the heater for burnt connectors or melted/damaged spots on the heater its self.
Does the pump hum louder than it used to? this may be a sign of a bad coil in the pump motor which will cause an amperage spike.
Realy sounds like the heater I would aggresively inspect the heater.