Tow Pro: Towing with a Pajero Petrol !!!


My question / problem has probably been asked many times... Sorry !!! We have just upgraded our Jayco Swan to a 18' single axle Paramount Duet pop top caravan, brand new and its a fantastic van. The wife & kids are very happy ! But I'm not. We recently towed the Duet to Exmouth WA from Perth a return trip of about 2,500 kms. I have a 2005 Mitsubishi 3.8L Pajero Automatic petrol. This was the first big trip in the van apart from a couple of smaller ones. Within the first couple of hours I realised the car was struggling in 'Drive' and it was automatically changing down, sometimes a couple of gears in order to pull the van especially on the hills. I eventually used the gear box in the manual mode and left it in 4th gear and sat on about 90 - 95 kph sometimes 100kph. This seemed to handle things a lot better. My question is: Is this OK ? will I doing long term damage to the gear box and will I use more fuel towing in 4th. Is the 5th gear an overdrive gear anyway ???? Any advice would help.

Bryan Morris, WA