If you have removed all of the cabinets, removing the shower surround itself is pretty easy. It is getting it disconnected that is the hard part. You will just have to drill out all the rivets, disconnect the supply water lines, remove the toilet and disconnect the shower P-trap and waste tank drain lines. Use care disconnecting the dump valve from the waste tank as these are expensive and hard to find. To remove the waste tank, cut the vent line that goes to the roof and unscrew the toilet flange from the surround. You will have to fabricate a tool to unscrew the flange seat that attaches to the waste tank. Repair the vent line with a rubber drain line pipe clamp that has 2 worm clamps. Also check the vent at the roof as with all this movement you will have just created a big old water leak on the roof.
The other part will be getting the belly pan off to get to the drain lines. (See my post to you on repairing the floor)
Work with care on the fiberglass, as their is no replacement showers available unless you find another old Bambi for parts.
-"You want to make it two inches - or, if you're working in centimeters, make sure it's enough centimeters for two inches."-Red Green