Aside from Alaska (which has some radar limitations due to the terrain) I'm pretty sure none of those accidents occurred after coast-to-coast radar coverage came in to force in the continental US.
Believe it or not until 1973! there were massive land-based lighthouses all across the United States that would illuminate the aerial pathways that aircraft would use to get across the United States.
"By September 10, search crews had found eight previously uncharted crash sites,[54][55] some of which are decades old,[56] but none related to Fossett's disappearance."
To be clear though: I was talking commercial operations which generally operate above the radar plane. I'm not even sure how/if Fosset's plane would appear on radar as he had no ADS and he didn't file a flight plan which means he was in class G or possibly E airspace on a VFR mission (very low altitude).
Believe it or not until 1973! there were massive land-based lighthouses all across the United States that would illuminate the aerial pathways that aircraft would use to get across the United States.
http://en.wikipedia.org/wiki/Airway_beacon