Hi Bill,
The custom map feature is for a smaller area than the ATR covers, for certain. You want a .IMG (vector) map similar to the one you used for Tour Divide.
You'll need to use Mapsource and some of the free US .IMG sources in order to do this (that's what I did for the TD base map).
It's a bit involved to make sure you cover the whole thing. Another option is to buy a DVD (or SDcard) preloaded with maps for the whole country. That's the more expensive but simpler option
Hope that helps some. Good luck out there this summer!