Uh-oh...
Nothing bonds permanently to polyethylene, so if the tank wall is cracked, the only option is replacement. But if it's only the weld that's failed, any shop that works with poly and has a spin welder should be able to repair it. Fortunately it's a water tank...'cuz I don't know of ANY shops that will work on a used waste holding tank. Look in the Yellow Pages for plastics mfrs...if you can't find one locally, I'll give the name of a company to ship it to...it won't cost you but about $6 plus UPS both ways, which isn't more than $10 each way for a tank that small.
