I have been fabricating and welding plastic for 20 years. Under the right conditions polypropylene is one of the easiest plastics to weld. A fuel tank, however, would be nearly impossible.
Plastic welding requires that all 3 pieces, the 2 that you are joining and the welding rod, reach melting point at the same time. The 2 pieces you are joining must remain still and pressure must be applied from the rod to force the resins to blend. With a tank the wall is so thin that it would collapse from its own weight about the time it gets hot enough to weld. Any pressure applied from the rod would only distort the tank or pop a hole.
Gluing polypropylene or polyethylene is a waste of time, effort, and glue.