Do you have access to a miter saw? If so, set the blade at 5 degrees and cut a 2x4 for shims. After the first cut, roll the board so the bottom is now the top and make your second cut. Repeat as needed. I do this in my line of work to create shims for the cabinets. If you can't get to a miter saw HD sells bundles of shims pretty cheap. Empty the tank, shim the end that is low, and check your level from side-side, front-back, and corner-corner in both diagonal directions. This will give you a true level.