I can't see any of our usual fountains of knowledge answering so here goes from a non mechanic....
the auto on the 150 doesn't naturally lock until you get to 2400 or 2500 RPM so, even though you have max torque at say 1800 RPM, the auto does not have the engine locked to the drive train and some slippage is occurring. Slippage in turn generates heat and heat is the enemy of the auto transmission. If you are not towing, and you are travelling along a flat road, even if the auto has not locked the heat build up is not too intense. As soon as you add the weight of a trailer/caravan the slippage increases and you get the heat build up. By the way, the same thing happens if you are travelling up-hill and your auto holds the high gear for too long (see other threads about this scenario)
Cheers
Mark
The easiest answer is because the owners manual says not to. If you ignore the manual and tow in 5th. and damage the gearbox the dealers will know instantly from the damage sustained and will walk away from the warranty.
Technically, snowman is correct, towing with a slipping torque convertor for extended periods will cause overheating of the box, will degrade the transmission oil and cause expensive damage inside the gearbox. You will also find that the gearbox will need to change to 4th. often and this will creat mors wear and tear as the box hunts up and down gears on every slight rise and fall of the road.
Driving in 4th. at 100 kmph you will rarely have any gear changes as the engine is at optimum revs for almost anything and because the gearing is optimum the torque convertor rarely needs to slip. All is happy and economy will not be affected.
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