Michael65, sorry, but in my opinion, any plastic welding will never work, especially because of the extensive damage your tank suffered. You will need to replace the entire tank replaced. Here in California, were I live, in Ontario, there is a factory which manufactures and sells tanks of all shapes. You will need to remove the old one and get the dimensions. They can custom make one for you, but it will not be an exact duplicate. Several years ago I bought a waste tank from them for my Class C, and it cost me around $150.00, I last checked their website and the prices are still relatively low. Someone else on this forum might be able to suggest another manufacturer closer to where you live.