Where to get fuel tank replacement ?

Good luck and be prepared to pay $$$$. Not many going to want to do major surgery on a part with so much liability on an old camper. 
They make gas tanks of all sizes. For RVs like this and boats. Sourcing the parts is not hard. And neither is install rocket surgery. But it’s an oddball request that you’ll have a hard time getting someone to fulfill I’m pretty sure. 

Time to start analyzing the value of the camper vs the amount of $ you’re sinking into a camper that is not worth much and in need of major repair.