Do you have any idea how big of a "crack" it has? Could it be like my fresh water tank where a screw holding the scissor jack rubbed a hole in it? There is one epoxy out there that might help if you have a hole or small area to repair.
Their is one epoxy that might solve your problem. JB Weld makes a Plastic Bonder Epoxy for poly plastic applications.
Plastic Bonder is a quick-setting, two-part, urethane adhesive system that provides strong and lasting repairs and works on thermoset, carbon fiber composites, thermoplastics, coated metals, concrete & more.
http://www.jbweld.com/collections/au...bonder-syringe
This is NOT JB Weld Plastic Weld nor any other JB Weld plastic epoxy product. Make sure it is PLASTIC BONDER.
I used a butane torch to melt some of the poly back into the hole, roughed up the tank surface and applied a couple layers of the Plastic Bonder. Then I covered the area with a 3 layers of different material. A layer of EternaBond tape, a layer of protective metal residing totally on the the EternaBond tape (my hole was on the edge so I use a piece of aluminum angle) followed by final layer of EternaBond tape to secure the metal protection in place. Then I made sure the sheet metal screw was smooth with the surface prior to remounting the tank.