The manage map dialog download whats on your screen basically. And it is really slow. If I want to download the entire US, I zoom all the way out until I can see the entire US 50 states, If I hit download everything for zoom level 6 there, it will simply crash my n900 because there will be 123,456,789 maps to be downloaded.

So basically, there is no way to predownload the maps for mappero. You must have an internet connection on the road, or you take very short trips and pre-browse the path you want to take and let mappero download it for you then.