Nissan Navara 2011: Is an external cooler necessary?

We bought a 2011 Navara Dual Cab STX diesel automatic that we often use to tow a 2.8-tonne caravan. We were assured several times by the dealer that an external cooler would not be necessary, but following your comments in Carsguide I wonder if that's correct. I prefer peace of mind, rather that a surprise along the way.