Been my experience that no tape or glue works on plastic gas tanks. I think most are made of polyethylene which seems to not let anything stick to it. Some glues or epoxies won't expand/contract the same as the tank and will just pop right off when there is a temperature change.
I've been trying out plastic welding on several items with some success although I've never been able to exactly reproduce the finish look without bondo/primer/paint. I have a small welder that looks like a 2 barrel electric soldering gun that uses wire that you press into the plastic. The hot wire melts into the plastic and is more for reinforcing where some strength is needed. You might be able to just use a pencil style soldering tool with the tip inserted backwards to melt and smooth out the crack that might seal up the crack. You can use a filler (like tig welding) but I think you would need to match the type of plastic. Mixing ABS, PVC, Poly, PEX, nylon or any number plastics probably won't work well. I've seen some use zip ties as a filler. Of course don't try this on a tank with gas or vapors in it.
IMO, best bet is a new tank but are they even available. The customer can't blame you if it fails. I've searched for some parts only find out they were poorly designed and the entire supply of replacements has already been snapped up.
