Though it may be a pain, drain it down to the bottom plastic trim.
Use that as a reference and shim it with "door shims".
Used for installing doors, they can be had in bundles at the big box home stores for cheap.
Once the tank is level on the trim, start filling it back up and use a tape measure or ruler at all corners to monitor it as it fills.
A hammer and chunk of 2x4 can be used to drive the shims in further as the weight increases if it crushes more and becomes out of level.
Once it's full and level, drive a screwdriver under any excess shim and pry up quickly to break off the stub, or use a hand saw to trim off the ends .