A higher tyre pressure will always improve stability so it is advisable to have the tyres at the pressure specified for maximum load when towing, even though the car may not be fully laden. Wear is increased slightly and the ride may be a little harsher but the benefits to safety outweigh these disadvantages, especially bearing in mind that mileage covered like that is usually only a fraction of the annual total. However, don't forget to readjust to standard pressures when not towing and not fully laden.