I've got an older gen taco and towed my 20' ski boat anywhere it was flat. As soon as there were hills, forget it...if you can make it up it you might not make it down. I think my boat weight is 2800 wet, and say 1000 lbs for the trailer, so maybe around 4000 lbs overall. It was painful and slow, but do able.
Also towed a 21' twin axle sleeper trailer, I think it was an old Nomad. That was painfully slow (like chickenhauler said, it was a brick)! I couldn't use 5th gear on the hwy, and stopped even slower. And I couldn't see anything behind me, legally I needed mirror extenders, and I now know why it is necessary.
Upgrade your brakes, Hawk pads made a difference. Not sure how the later gen rear frames are, but I plated my frame to add support. Upgrade your rear leafs or add air. Make sure your leafs have the heavy duty leafs, I know in 05 some tacos had them and some didn't. I think you can tell by the number of leafs. I'm assuming you have the 6 cyl motor, is it a
manual or auto? Are you planning on doing many grades or primarily flat land?