Wow - in this case there is no quick fix - unless you look at a hunk of plywood and set a porta potti on top - and replace the belly skin.
To do that job right you need to remove the bathroom - or at the very least the toilet section completely - pull the floor in that area - have a welder come up and get you sorted out underneath.
Either fix the fiberglass Blk tank or get a new poly one made.
What could damage it so bad???
TIME WATER and ICE! Looks like the bulging in that tank is suspect of going to bed full for the winter - I would have hated to be where ever she sat when she blew - at least it would have been frozen but the spring would have brought a huge mess.
Corrosion look just like acid - pitting spots resemble acid spills but it is simply water sitting or dripping over time.
Good Luck with the repairs.