Hi Tina
The heating we have is the Truma type with the blown air switch on the heater and the control switch on the front bed bunk, it has a black dial numbered 1 - 9 and settings of 500w, 1000w and 2000w.
After speaking to the dealer and a bit of investigation, I have found that like you, I sometimes need to turn the thermostat up to 9 to get it to come on. If you have a Ranger 550, there is a further air temperature sensor mounted at the base of the cooker, this connects into the back of the cooker via a jack plug very similar to a headphone socket. In my case this plug was not pushed in properly.
I understand this sensor is optional and although I have not tried it, I would guess that because its optional, if you unplugged it, the heating should still work but rely on the main control thermostat for temperature control.