I like mysteries. There is no way we can know what happened exactly, but I can think of a few reasons.
The reason things do not make sense is usually we are missing information, and what we presume is not correct.
The facts-
He is the driver of the vehicle.
No drugs history.
He has a family.
He is in a national park.
He said he's going to jail.
He was not asking for help or shouting.
He disappeared shortly afterwards.
His skull was later found a short distance away.
What we can deduce;
He has a photo of his kids in his wallet.
He is on a route where hitch hikers are often picked up by large trucks.
Hitch hikers often know the route, and stay in the forest.
The sleeping bunk is behind the driver and covered with a curtain.
He carries a shovel on board in case of snow or getting stuck.
He did something illegal to make him think he's going to jail for it.
So a typical scenario may be that he picked up a couple of hitchikers, one pulled out a gun and took his wallet. Seeing he has family and his drivers licence gives the address, this was then used to make him do something illegal- possibly even killing someone. They could easily be in the bunk behind him with a gun, saying if he did shout out they would kill him and go after his family.
They then left and forced him to drive deep into the forest - with the body in the back and after both of them carried the dead body to bury it, he was also shot and buried in the same hole.
The person responsible then mingled in with other campers in that area.
Because the hole was not very deep, wild animals were able to dig up his head at least and leave it on the surface to be discovered.
I think there was a reason he drove to such a remote area, and he was made to do it.