In My Side of the Mountain, who was the only person who believed Sam could live alone?

I believe that the answer you are looking for is Miss Turner.  In My Side of the Mountain, she is the only person at the beginning of the story who really takes Sam seriously.

When Sam runs away from home, he hitches a ride with a truck driver up into the Catskill Mountains.  When the truck driver drops Sam off, Sam tells him that he is going to live in the woods.  The truck...

I believe that the answer you are looking for is Miss Turner.  In My Side of the Mountain, she is the only person at the beginning of the story who really takes Sam seriously.

When Sam runs away from home, he hitches a ride with a truck driver up into the Catskill Mountains.  When the truck driver drops Sam off, Sam tells him that he is going to live in the woods.  The truck driver tells him that he will be back in the afternoon in case Sam needs a ride back to the city.  That shows he does not believe Sam will be able to live on his own.  Next, we hear that Sam’s father had laughed at him when he said he was going to go live on the Gridley farm.  He thought Sam would be like he had been as a youth when he “ran away” and was back home in bed before anyone knew he had left.  Later, when Sam gets his fish cooked at the home of “Bill something,” Bill tells him that the door will be unlocked for him if he needs to come back that night.  Again, this shows that Bill does not really think Sam will make it out in the woods.

But then Sam goes to the library in Delhi.  Miss Turner is the librarian there.  When Sam tells her what he is planning to do, she does not laugh at him or say anything to make it seem as if she does not take him seriously.  That is why Sam says, on p. 22, “Miss Turner was the only person that believed me.”  Therefore, Miss Turner is the best answer to this question.