Is your beast a diesel?
When was the water pump last changed? These can get "loose" so the pump is being turned by the cam belt but the impellor inside the head is not turning at the same speed.
This needs a new pump.. but it's best done at the same time as a cam-belt change to avoid extra work... unless it's failed, obviously (in which case do it asap... but also fit a new cam-belt so it's all done for a few years).
Also.. try bleeding the radiator (stone cold engine). If there is a screw on the back (left or right side) unscrew it slowly until either air or coolant comes out (don't take it out completely as it will be tricky to put it back in when coolant is leaking out
) Tighten it back up when coolant comes out. Repeat after a few days,
If you keep getting air in the rad' then that could be a head gasket problem but the diesel isn't particularly prone to this.
The thermostat should begin to open when the beast gets to 86C and be fully open when it reaches 90. 90C is usually the "big" or "mid" mark on the temperature gauge. The cooling fan should come on at 92C and stay on until the temp drops to 90C. If your temperature creeps up towards 90.. drops a bit just before it gets there... then rises until just over, when the fan comes on.. the thermostat and fan are working properly.
If the radiatore is in good condition and the pump is new and there is no air in the cooling system and the fan comes on when the temp gets to 92.. then the beast should stay cool (that is below 92C).
Towing up hills will make it run "hot" more of the time than when you're not towing but the beast should never usually overheat so that the light comes on... so check all of the above. I only ever saw the overheat light when I lost a head gasket once (air in the rad' was preventing cooling).
Ralf S.