Operating Temp when Towing

Operating Temperature
I have just purchased a new Touareg V6 180. Have done 4500km now. I recently towed our caravan which is not big 15' to South Australia a one day 800km trip approx. it was a warm day 37c degrees plus. I noticed the temperature gauge that always sat at 90 degrees continually fluctuated between 90 and 105 (still in the normal range but at the high end). I have never experienced this with my other tow vehicles (Land Rover discovery & Nissan Pathfinder). I was towing at 100kph. The car was fantastic and did the job with ease, but I have a question re the temp gauge movement. Is this normal? Have other owners who tow experienced this. The manual does say the vehicles operating temp will be effected by outside ambient temperatures. Advice from the technical people at the VW dealership say this is normal, but as I have said I have not owned a vehicle that has had the temp gauge continually go up and down like this? I would appreciate feedback from others who may have experienced this. As I purchased the Touareg primarily for a tow vehicle and will probably being towing when the ambient temperature will be 35c plus often, I would like some assurance all is ok?