Hello again Joinery,
I agree with you and Seth, 17mpg is low.
I run a VW Passat (56 plate) with the 170bhp PD 2.0 litre with DSG 6 speed (roughly equivalent to the multitronic) box, But I don't tow with it. So I can't make a direct comparison. But what I have discovered and I suspect it will apply to the Audi also is that it seems quite sensitive to service intervals. Keeping both the engine and the gear box properly serviced does make quite a difference to the economy of the vehicle.
What I also suspect in your case as the vehicle is much more recent, it has probably been tuned for economy, and to achieve the prodigious claimed MPG's and the CO2 emissions,it will be more sensitive to extra loads than for example my older Passat.
I suggest you discuss the issue with your garage, who may be able to do some checks to see if there is a problem.