Buy enough vehicle to trailer what you want, or buy a small enough trailer to work with the TV you got, without crowding the margin.
Really, it's that simple, leave a hefty margin of tow capacity, don't get near max, and you won't have problems.
Touareg is rated for 7700 pounds towing and 1300 pounds in the TV. The TT I bought is 3700 dry, with a tongue weight of 320 pounds. That's one of the main reasons that I bought it... well within the tow vehicles limits.
Use a little common sense, and a test drive is not even a consideration. I drove out to Indiana to pick up my TT from Los Angeles, because I can trailer for a heck of a lot less than $ 2.50 a mile, certainly when the Touareg gets 33 -35 mpg empty, for half the trip there. Never even considered a test drive I was well within the numbers. Did install an Equalizer hitch at the shop.
I believe it's called common sense, don't tow near the margins of your capacity limits, and all is well. Trying to make silk purses out of sows ears, or going over the limit, or even near is, is lacking common sense, with little margin for error. That big margin of error, or buffer zone is your friend, not something to be tested.
Do the math ahead of time, a test drive will tell you nothing.