Frankly, with so many toolboxes I think you could put gas cans in them and with cutting hole in the deck, have sewer extender under the bed.
This way you will move whole COG forward and avoid the possibility of doing the job again, when you find you have too much behind rear axle.
This way you can also do some existing hitch reinforcement and with no need for stinger -save $2000 if not more.
Bare in mind that in frontal accident - campers move forward and tilt to crush the cabin. Some more than wood in front would be advised.
For the ties - can you run them inside the boxes?
