Nissan 4.2 Turbo Diesel Gearbox Failure

I have a 2000 Nissan Patrol 4.2 Turbo Diesel and tow a 22ft Caravan regularly. The 5th gear failed completely at 62000Klm and when it was picked up the mechanic was not surprised and advised me it happens quite often to the patrols that tow trailers at approx 60000klm. When it was repaired ( under Warenty ) the service staff then advised me that they had replaced the main shaft and 5th gear with a updated version but I should not tow in 5th or travel with the car under stress in 5th gear , eg up hills or at less than 80KPH. It appears Nissan staff are aware of an problem with the 5th gear but just forgot to tell me the important restictions when purchasing the vehicle even though I had selected it for its towing capacity and power at the time. Has anyone else had there gearbox fail in this way and what was Nissans response? Also has anyone been advised by Nissan of any restrictions in using 5th gear.