how did you manage to get the route displayed on the map?
The road is described as a kml file.
Gmaps has a parameter named kml that accept a csv list of KML or GeoRSS files. e.g: &kml=`usa-1,usa-2,usa-3` All these files are added as overlays on the map.
To set up the trace, I define the road with google maps. Then, get the link to the map (button on the upper left side of the map). copy/paste this link in your browser with &output=kml then you will get a kml file.
If the trace is too long Google probably cut your trace. So you probably need save several traces.
Open the kml file created by google maps and remove all unwanted tags to get a light file like this one:
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.2">
<Document>
<name>USA 99</name>
<description><![CDATA[National Parks - 1999]]></description>
<Style id="style5">
<LineStyle>
<color>7f0000ff</color>
<width>4</width>
</LineStyle>
</Style>
<Placemark>
<name>From Las Vegas to Salt Lake City</name>
<description><![CDATA[]]></description>
<styleUrl>#style5</styleUrl>
<LineString>
<coordinates>
-115.148750,36.085941,0.000000
-115.148033,36.084648,0.000000
-115.147461,36.083900,0.000000
... about 19630 coordinates are stored in the kml file
-111.891037,40.760738,0.000000
-111.891037,40.760738,0.000000
</coordinates>
</LineString>
</Placemark>
</Document>
</kml>