Most TCs list the overall length, My guess is you'll need about 18-24 inch extension. Where the ball is impacts the felt weight from the tongue. I'm assuming you have the short bed. From what I understand the Megacab is only offered in short beds.
Fwiw, you are likely to be overloaded without the boa by the time you add water etc. You have to include the tongue weight in your payload which is probably 10-15% of the 6200.
If this is a long term plan unless you only plan to short tow it, you'll likely need a DRW. Another option if close by is to take the TC to the campground drop it, and go back for the boat. Of course, you could do that with a TT as well.