Without getting into the reduction is density due to the density of excrement being (generally) lower than water, the general rule is that Water weighs 62.5 pounds per cubic foot. A the bottom of a 2' tank, the load on the sides of the tank are 130 psf (at bottom of the tank). Since you might have the height of the water in the hose to the vacume breaker, it might be more over 4'. That way you'd have say 245 psf along the bottom edge tapering down to over 130 psf load on the "top" of the tank. Then, if you start to add a little vigourous pumping pressure (before you empty the "full" tank), very shitty things can happen to your cabinet. It would certainly be memorable ;^)))

By the way, 14.5 psi would equal about 1,300 pounds per square foot (or 144 square inches/square foot x 14.5 psi)