IMO, if TH had parked on the access road and was leaving about the same time the GK reported that he was leaving (8:45 - 9:00 a.m.), they would have likely seen one another - even if for just a second or two - especially if TH had to back up all the way back down the access road to Skyline Blvd. as the GK loaded up his mower onto the trailer.
Since the GK has stated that he didn't see the Horman truck on the access road - I'm inclined to believe that the Horman truck was not parked on the access road.
MOO
That, and it just doesn't make any sense to me that someone in the commission of a crime would park a bright white truck in a spot that imo is very conspicuous from Skyline Boulevard during a time when parents, school buses, passersby, were traveling on Skyline Boulevard. That's the last place I'd park if I wanted to be inconspicuous.