Well you ain't going to like this.
You have to remove or at least lift up the entire rear body section to get the tank out.
The rear section is bolted to the frame under the operators console ( turret to some ).
That will require removing the battery & battery support then undo the 4 to 8 bolts that hold the turret down.
Get under the mower & remove the steering gear from the end of the shaft so it can be pulled up.
Remove the seat and the seat springs which also hold the rear body to the frame.
If yours has levers poking through the guards set them to a position that will not foul as it is lifted.
Raise & choke , block , hang the guard as high as it will go ,
Remove the fuel outlet from the tank then wiggle it out on the right side.
You can just winch up the rear after removingthe seat & springs but that always puts a couple of creases in the foot boards where the turret clamps down on the real body pressing.
This compromises the coating and the strength of the body pannel which will start rusting around the creases.