How can I save Google Maps in iOS for offline use? Is there a way to retain a map with as much detail as possible (street view, maps, directions, zoom levels) offline? 

This question refers to the Apple maps program that shipped with iOS versions 1 to 5 that relied on google map tiles. Please open a new question for iOS 6 maps and the google map app that is also available on the App Store since 2012.
Without jailbreaking, the iPhone DOES cache the last few map tiles on the view, at all the view zoom levels. I'm not sure how extensive this is, and because it's only a temporary cache the data is always a second away from deletion. 

Without jailbreaking, you can download an application, there are lots of them, but my favorite is called MotionX GPS, which is on the App Store. It allows you to cache and save certain types of map tiles from a variety of map modes, including Terrain, roads, and more. I'm not sure if they're specifically Google Maps, but you can get some types of maps for sure.

After jailbreaking, which is both legal and easy, you can find a couple of 3rd party utilities to help you download map tiles to the device, but most of them require you to view the tiles in the app itself, not in the Maps application.