I also picked up a RAM yoke mount for my iPad 3. I've not tried it in the 172 yet, but will hopefully do so next week - planning on mounting it on the RH yoke in landscape. Seems like a great mount.
You can use the iPad GPS without any cellular connection. Just make sure that cellular data is set to off in the settings app. I also set Bluetooth and wifi to off, this will give you longer battery life. Enabling 'airplane mode' however turns everything off including the GPS.
Also, if you are buying an iPad, make sure you get the wifi+cellular model as the wifi only model does not have GPS.