A possible solution is to use a heightmap of the usa, for example:
However this may be a little big anyway you could then read this image pixel by pixel creating a tile with that relative height for that shade of color.. so say White (highest) and black is 0 (Lowest / Sea level) producing an accurate height listing of usa.
since you now have a array of heights you have no issues on the angle of isometric tiles
It is important to remember the HeightMap above is not seen in your game but read by your game to generate the tiles. (In this case this heightmap would need to be rotated by the angle of your tiles (i.e. like 45 degs)
When creating isometric tiles all the tiles ideally are the same angle rotated.
The Red/Blue line that intersect in center of the tile is the "Plane" the maps 0 height reference so in this case your sea level could be at this height. as the shade gets lighter you could increase height however this will make the terrain impassible as there no way up. To rectify this you would use a slope tile which is a block which is designed to look like the slope and can not be facing directly towards the viewport.
Here is an example of all I describe but you have to use your imagination to see that even tho it looks like blocks once textures are on it will look more realistic.
I was digging around and noticed Age of Empires 2 using a similar method to above (increasing the y coord of the tile) but also uses shading to darken inclines works well i think.
Hope this helps