Counter weight for added back storage area/deck

I can believe that fully loaded platform needs a counterweight. But I would find something useful to use as a counterweight, maybe an extra fresh water tank, rather than concrete.

Load the trailer as usual, get a tongue weight with the concrete in place. If it's more than 15% of the trailer weight, start removing the concrete until the tongue weight is where you need it. Shoot for something like 12-13% of the total trailer weight as tongue weight. All weights should be loaded ready to roll.