Not too sure about any free Boondocking locations along the route you mention. However, Homolovi Ruins State Park (Arizona) has non-electric camping for about $10-$12 per night. They also have electric sites, but there about double the price.
This is a decent stop-over not at all crowded just north and to the east side of Winslow, AZ.
Take I-40 Exit 287 and go north about a mile to Winslow-Polacca Rd. then about another mile to the park. You'll see the signs.
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