It seems most people seem run a separate earth cable but you really do need to consider the distance. If you are doing a long run to the back of the car then the resistance can be quite high. In this case, the car body will offer less resistance. Alternatively you would have to increase the wire gauge to reduce the resistance.
However I wouldn't use the towbar as the earth point because you don't know what the connection to the chassis is like (Loctite on the bolts for example). There will be a chassis earth anchor near the rear. On the latest model there is an anchor behind the rear left hand trim.
The following article explains some of the issues if any is interested.
http://www.w8ji.com/negative_lead_to_battery.htm