Ok... so I went through this same mental exercise (and exhaustion) before towing a 24' box trailer weighing around @ 5,500
lbs over 1500 miles (nearly non stop).
I have a 2013 Touareg TDI with factory hitch. I presume we are talking factory hitch.
The real question is what does your trailer weigh as you plan to tow it?
The next question is what is the actual tongue weight (I bought two 500 lb digital bathroom scales on Amazon to find out... I only ended up needing one).
One thing to note... I found a HUGE difference in weight between the jack and the wheel. In my case I built a cradle out of some 2 X 4s to evenly spread the weight on the (glass) scale top. I then used a piece of wood under the ball coupler with the trailer sitting approximately level. I measured around 400 lbs actual tongue weight.
As far as the 10% or 15% rule... well it is a rule of thumb, and technically until 2014 or so there was no official US standard for trailer tow ratings on SUVs and light trucks. it was purely up to the manufacturer to define their own tow ratings, thus it was difficult to compare. The standard today is SAE J2807. The SAE J2807 was created to help consumers compare tow ratings on light and heavy duty pickups. Not sure if the Touaregs were ever rated or evaluated to be complaint with this.
You asked where the tongue rating came from on the hitch... my guess is that VW down rated it from the 770 of the 2010/2011 hitches by subtracting the weight of the hitch itself from the rating. I almost bought a 2011 hitch to put on mine but found it had the wrong part number. After a lot of research I am 99% sure the hitch metal is the same, but the difference in part numbers (between a 2011 and 2013) is a different decal for the tongue weight rating.
When I bought my factor VW hitch to have installed on my Touareg it weighed around 80 or 100 lbs when UPS dropped it off at my house. In all honesty it was built heavier duty than an OEM GM hitch on a 2003 Chevy 2500. To say it is over engineered is a compliment in my book... but it is heavy. It also attaches with ( 8 ) 12mm bolts (not really relevant to this discussion except to say that it is well fastened).
VW actually rates the tow rating of their receiver by keeping the trailer ball center-line rather close to the hitch assembly. This is how they get such a big rating on it. Most aftermarket hitches in the US (especially those with 2 5/16" balls) have the ball center-line further from the hitch. This changes the loading and rating slightly (lots of math involved).
The simple answer is to use the VW hitch with VW receiver and 2" ball if you plan to tow a trailer close to the max weight rating of the Touarge. I've towed smaller trailers with after market hitches and don't think twice about it.
As to how to connect it... I'm not sure. The 2" ball couplers should be rated up to 8,000 lbs or so. Though VW only rates the ball to 7700 lbs or perhaps 8000 lbs (I don't actually own this receiver). According to West Marine (https://www.westmarine.com/WestAdvis...s-and-Couplers
), a 2" coupler is good to 8,000 lbs. If the trailer you buy has a 2 5/16" ball it may be over kill. You could look into replacing the 2 5/16" ball coupler with a 2" one (though I'm not sure that is the best option).
On the other hand it could also be designed for tow by normal pickups where a 2 5/16" ball is the 'camping' standard on bigger trailers.
Another option on heavier trailers is to use a weight distributing hitch (my recommendation).
In my case that is what I did with a 2 5/16" ball and 600 or 800 lb WD distributing hitch springs. When I first hooked up the trailer (note the ball cener-line is quite a bit further than VW recommends), the rear squatted quite a bit. Once I set the WD springs the rear end lifted almost to normal height and the front went down. I was nervous the first few hundred miles but after 1500 miles it was no big deal. In fact the Touareg pulls very nicely configured this way.
The easy answer for you is to drive your Touareg to the trailer you are considering, hitch it up let the tongue weight down and see how much the rear squats. If you feel uncomfortable with it consider a weight distributing hitch as I describe above. If it doesn't squat you might consider driving it. If you can use the factory 2" ball and receiver tube you may get slightly less rear squat.
Really it all comes down to the weight of the tongue and the trailer. Remember where you place the scale can significantly change the tongue weight reading you see....
The scale I used was from Amazon and was the "EatSmart Precision 550 Extra Wide Digital Bath Scale w/550 LB Capacity Stainless Steel ".
Also I forgot to mention I use a Tekonsha P3 digital electric trailer brake controller mounted under the dash to the factory electric brake controller wiring.
As a last note VW tends to over engineer and be overly conservative in their manual.
Take a look through the posting of tow rigs: https://www.clubtouareg.com/forums/f...57125-111.html
Post #1105 shows a bit of the squat I talk about. I didn't like that much squat especially on a 1500 mile trip, so I use a weight distributing hitch and you can barely see any squat at all in the rear.
With all that said, can you post the trailer weight?