Wheel simulators, tire valve extensions and other questions

There are very few epoxies or other adhesives that stick well to polyethylene, which is the (translucent) plastic most RV water tanks are made from. Fittings are generally welded in place at the factory, from what I understand. If enough of the cracked fitting is attached and it's big enough, perhaps you could drill it out and tap it and thread in a nipple or something.