Thats a tough one did mine last year. Remove the tackle drawers under the chair pedestal and you can gain some access but its impossible to reach the drain on the smaller box (you can get to the big one from the access plate by the 2 thru hulls) so you have 2 choices in my experience, make a hole in the smaller fish box and put in an inspection plate.
OR after you put in new thru hulls plug the smaller box and use it for dry storage, i keep my lines in there, and run the sink drain directly to the redundant thru hull.
Its a tricky job and dont force the drains on the bottom of the fish boxes they break pretty easy, heat gun or a hairdryer helps make things easier if the rubber pipe is really old.
If you find a better solution keep me posted or if you make hole let me know where was the best position. I'ts on my TO DO list this winter.
Good luck.
