I've been on the search for a GPS and as 1Louder stated, a tablet seems to be the way to go. The versatility of a tablet, combined with the plethora of available navigation apps geared toward off road use, the ability to monitor your rig's vitals and a 10" screen if you so desire, and it's a tough option to pass up.
The hard part, as I've discovered, is finding a navigation app that works for you. There are a lot of them out there.