Having now had a chance to tow our 20'T/A caravan with the S-max, I would not recommend towing in 6th, whilst the engine is capable of holding 60 on the flat, it uses more fuel and places excessive strain on the engine when producing near maximum torque for long periods.
We found 4th to be the most economical gear for towing, sometimes selecting 5th or 6th for long, shallow decents.
We were right not to bother with the self levelling suspension option, with the van hitched on and a bootful of kit, the rear suspension barely moved.