We've just got back from Spain. Towed with A6 2.5 diesel Automatic (tiptronic). Usually tow with manual Landrover Discovery. Obviously A6 is not has heavy as Landrover, so had to lose a bit of laden weight. Averaged approx' 24-28mpg on toll roads etc and against some very powerful headwinds at time with a big twin wheeler caravan.
Depends how old. If you are looking at a 1994-1997 model (A6/100), test drive thoroughly as the 'box can be fragile, and the car will be getting on a bit by now as well which doesn't help. A6 Mk2's 1998-2005 usually bulletproof but the tiptronic box is probably stronger for towing than the multitronic CVT, albeit less economical.