My boat wts. about 5000#. I have a Dutton Lainson(DL) winch mod. # SA 9000 & it pulls the boat on with no problem. The trailer is a roller/bunk combo. DL. also makes a SA 12000 model winch. You can also double the winch cable to the boat with a pulley on the boat & back to do winch anchor post which will increase the pulling power but cut the forward movement in half.
On my power cable from the battery I installed a 12v outlet with spring loaded outdoor cover & mounted to the hitch frame,. From there to the winch I made up an extension cord in between. I carry the cord in the truck & plug it in & out as needed to load the boat. Just an idea.