Looking for a GPS with high detail zoom level

I'm having big problems finding a suitable GPS for exploring very unpopulated parts of the world that doesn't have major roads.
Then I zoom out to see a whole country (500km) I would like to see even the smallest roads if I want to, not an empty screen.

We have always used Streetpilot 7200 in our expedition vehicles, they allows you to set zoom levels manually on every object which is what I want.
But they are getting old and have very low resolution.
I've tried buying a Garmin 2797 but it's just as bad zoom level as the rest modern Garmin I've tried, the smallest roads disappears after 0.8 km zoom level at best!

It must be Garmin maps compatible, I want to use OSM maps etc. I would prefer a big screen 7-8" and a GPS that can calculate routes.
Any suggestions? Hack the Garmin 2797? Any good app for Ipad? Or Android? Or Win 8 tablet?