After a long drive in hot conditions the gears would not kick down on a rising gradient. after struggling for 20 minutes driving it went in to limp mode. After stopping the computer reset and seemed to be ok for a short time then the problem returned. Had the car towed to specialist. Next day car behaved normally. No codes came up and there does not seem to be a problem. There has been no indication of a fault in the past. I am concerned that the problem will return at some critical time in the future and let me down. What do you reckon?

have vehicle scanned for reason it went into limp mode and shut down

It's going into "limp in" mode. Get the trouble codes read.

check for broken motor mounts. if it is a rear wheel/4 wheel drive, check your u-joints in your driveshaft. if the aforementioned is good, check the fluid level and condition in your transmission. last resort, if rear/4 wheel drive: check your rearend fluid level and condition.

DIY Crankshaft Position Sensor BenzCranks No Start
Problems with the Crankshaft Position Sensor (CPS) on Mercedes-Benz cars are fairly common. If you have been experiencing stalling or no start conditions, a failed crankshaft position sensor is often the problem. Your CPS may have failed if the engine cranks but it will not start. At first, you may experience no start conditions intermittently. As the crankshaft position sensor fails completely, the car will no longer start or will die when you are driving. Starting problems are initially random, but more frequent when the engine is hot. Other symptoms of a failed crankshaft position sensor are poor acceleration, engine vibrations, poor throttle response, bad fuel economy, hesitation, engine stalls. The engine starts if you let it cool down. (Thanks to MB Medic)

Perhaps you're over working the truck. Up a hill with a load, heavy load? Driving in the wrong gear with a load maybe.
If the truck does not already have one you may want to look into an oil cooler. And if it has the factory cooler maybe a larger one.

The fan is only supposed to come on when there is excess heat .. in normal driving conditions it doesn't start. When you start your car in the morning the fan won't come on because the engine is cold.

With a hot engine - and it depends how hot the engine is - the fan will only kick in to prevent overheating. In heavy slow moving traffic or a traffic jam, the fan will kick in if needed.

The air flow through the radiator is usually sufficient -in normal driving conditions - to cool the engine.

When you park up and the car has a hot engine, the fan will kick in. This is perfectly normal. It is caused by a surge of hot coolant triggering the fan sensor. Perfectly normal.

The fan then should cut out after a while .. maybe 2-3 minutes at the most. If the fan runs continuously that suggests a fault in the temperature sensor switch.

Next time - wait by the car and see how long it takes for the fan to quit after you have parked up. It should stop of its own accord after a couple of minutes.

If the fan cuts off after a short while, and your battery is becoming drained, that's probably a battery or alternator fault.

Remember: when you start the car the fan won't kick in at idle speed because the engine is COLD, and there is no reason for the fan to start up ..
The fan WILL kick in for a couple of minutes after you have parked the vehicle and the engine is hot

If you drive your car more likely for short distances then engine will not get hot enough to start the regenerating process to clean the cloged particulate filter resulting in a antipollution fault.

Drive your car to a motorway or empty dual carriage way (best in nights). The temperature meter should be showing above 60°C when you begin with this process. Place 3rd gear and drive your car at 3000-3500 rev /min for 15 to 20 minutes to let the ecu to kick the regeneration process. Keep driving car for 15 minutes in this state will be enough to clean your catalytic converter and your problem will be solved. But if it doesn't then the problem is with your catalytic converter and more probabily you need to change it.

If check engine light is on ,remove battery cable, you have an electrical problem, maybe fuse?

if it did not travel any further than you say and you were driving down the road as this occured i would have to say i sounds like your thermostat is sticking open? if this was occuring at idle it could also be the fan is running constanly, please check the condition of your coolant and hoses if they are soft and old i would change them as well as the thermostat and take it from there these parts are inexpensive and would be considered maintance as well good luck and please rate -jeff

Can be many things. Cheapest would be the thermostat. Then the waterpump, most expensively would be the timing belt. 200,400,1100.