Hi Joe: Demolition! Now thats the fun part!! (smile)
You are correct, those one piece fiberglas units are always installed and the room built around them. I've never seen them come out in one piece.
Best tool would be an air chisel. Make sure there is no plumming running along side the tub underneath. Then start at the front, knock a hole to get the edge of your chisel into and zip right across. Then do the same thing going the long way, taking the unit out in quarters. It'll cut like butter.
If you don't have compressed air, a roto hammer with a chisel point will do.....just a bit more work Last ditch would be a recip saw or a jig saw, but the blades will hit something all the time, and you'll probably go through a few. Also, avoid cutting into the wall.....don't want to it something you'll regret :-)