AT&T Connected Car

You can search the forum for Mobley (the device that works like a Mifi) or take a look at this thread for an overview. As mentioned, it's unlimited data for $20/month subject to network prioritizing "IF" on a congested network after 22GB of usage. Thus far, users are not seeing any noticeable throttling.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29200191.cfm