Noting whether or not it leaks
when close to empty or not will give you some indication as to where the leak is. Obviously, if it's a problem with the vent or fill connection, it will only leak when full. If you can't visually tell if the out take connection is leaking, throw a towel under it to see if it's leaking.
Are you sure the leak is at the tank? Could also be at the pump, etc.
You might be able to repair a cracked tank in place, but likely it will need to be pulled out. I know some boats actually require one to cut the liner to get at the tank. I known some people have abandoned their original tank rather than cut up the liner.