I agree the tongue is an optimal place to mount a spare. I do have Scott's original tire carrier from his TW and hope to have it mounted as on mine by the weekend (Thanks, Scott!
) It's designed to be welded to the rear bumper, that's what Airstream did for my particular model in my particular year. That doesn't necessarily mean that was the *best* way to do it. But after looking at pros & cons for my trailer, I've choosen to install it the way it was originally intended.
We've all seen Airstream design issues that made us scratch our head and seen changes or improvements over the model years with some. The point is to research and evaluate the risks vs. benefits. Consider modifications involved and do you have the resources to carry them out. What will give you the most bang for your buck and what could cause problems with your trailer design, i.e. rear end sag, weight distribution, etc. Like Silvertoy says, a spare is imperative, a must-have item. Don't leave home without it.