I'd check a couple of things before pulling the tank, assuming yours has the General RS800.
An RS800 inspection guide lists a missing spring and retaining nut on the inversion rod as a common cause of trouble selecting reverse. The rod is at the front of the transmission. Might be worth checking that the spring and retaining nut are in place, and that the rod is seated properly in the inversion lever if you can see that end.
If those parts look good, then pulling the tank to look at the controls on top might make sense.
When you try to back up, is it completely quiet, or do you hear any grinding or belt squealing? Does it try to move at all?