Burning smells from 6 speed automatic when towing up hills

When towing my caravan that weighs approximately 15 100 kilograms, smells like my transmission is burning, when I put it back to fifth gear the smells disappear have I done any damage to my transmission and do I need to get this transmission serviced regularly if I tow, Ford has advised me that my 6 speed automatic transmission has a external cooler and as far as servicing my transmission is sealed for life, l do about 2500 kms per year of towing