Prado transmission cooler

When I asked Toyota to fit an external transmission cooler to my 2008 Prado diesel I was told it wasn't necessary as there is already on in the radiator. I then had an automatic transmission source one with the biggest cooling area he could find in Perth and fitted that. I subsequently towed an 18-foot Jayco Heritage from our home in Busselton to far North Queensland via Darwin without any increase in temp, in fact the temp gauge sat lower than it did before I had the transmission cooler fitted. I recommend an external air-cooled transmission cooler and to change the oil and filter at least every 40,000 km. We know heat can kill transmission and with no way of checking oil smell now Toyota and others have stopped fitting transmission dipsticks we must look after our trannies.