There are lots of ways to figure because everyone loads a vehicle and trailer differently and there are lots of different types of trailers. Someone with a large family in the TV will take up most of the payload before then even connect a trailer. Someone towing a trailer with a high percentage of tongue weight will run out of payload before they run out of tow capacity. If you are towing something like a boat with a low percentage of tongue weight you would likely run out of tow capacity before payload became an issue.
You have to calculate things based on your situation.
Basically GCWR - loaded vehicle weight (not the GVWR mind you, that actual weight) = maximum trailer weight. However, if you load the TV heavily you may run out of payload to support the tongue weight of the trailer. So yes, your 14,000 - 6,000 = 8,000 is correct. An 8,000 pound RV will have about 1,000 pounds of tongue weight which, when added to the 6,000 pound vehicle puts you right at the 7,000 pound GVWR. You would need a WDH to tow that heavy.