Also make sure that your throttle body is clean, air filter is clean, perhaps replace your fuel filter. Also check fuel pressure, I have heard that this can cause fuel consumption issues also.
Coolant temperature sender is located just under the thermostat outlet, its the brass coloured piece, not the temp gauge sender, which has the small green wire attached to it. Only way to test it is to check for fault codes, or hook up the car to EFI Live and view the temperature from there.
Another thing to check is the thermostat itself. I've just replaced one on the parent's VS V6, temp gauge was sitting very low, and heating wasn't too good either. Engine ended up running too cold, ended up doing 14 litres per 100kms, all freeway travel!
With the thermostat replaced, economy was improved no end, averaging around 8 litres per 100kms again all highway driving.
I've also put some Holden Top End Cleaner into my fuel tank, which helps by cleaning the fuel injectors, and removing all the carbon and crap in the upper part of the cylinder and valves...