OP implies he knows how many degrees off plumb....
First measure the distance between your side-to-side wheels and the front-to-back wheels (pick center-lines to be really precise).
For side-to-side (probably about 90 inches) plug this into Google "arcsin ? degrees * 90 in" filling in the ? with the number of degrees you measured and Google will tell you how many inches to shore up the side-to-side.
For front-to-back (probably about 26 ft on your rig) plug this into Google "arcsin ? degrees * 26 ft in in" again filling in the ? with the front-to-back degrees and Google will tell you the answer in inches (that's the "in in" part) to shore up the front-to-back.
Now you have the exact amount.
Google using the spaces just like shown and change the 90 and 26 to whatever you actually measured.