Why does Maniac choose to sleep in the equipment room instead of accepting the offer to share Grayson's room in Maniac Magee by Jerry Spinelli?

Maniac is used to fending for himself, so he wants to sleep in the equipment room.

By the time Maniac met Grayson, he had lost two families.  The old man found him sleeping in the buffalo pen at the zoo, and Maniac was a curiosity.  He made sure that Maniac was fed and cleaned up.  He took him back to the equipment room behind the band shell, but did not feel comfortable leaving him there.

When Grayson offered to take Maniac back to his room at the Y, Maniac tried to convince Grayson that sleeping in the equipment room was not that bad.  He didn't want to explain to Grayson that he had “bad luck” with parents and should “stick to himself.”  He didn't know how to tell the man without hurting his feelings.

"Oh, it's not so bad here," he said. "Look -- " He lay down on the chest protectors and closed his eyes. "Ah ... just like a mattress. I can feel myself dozing off already." (Ch. 24) 

Maniac also asked Grayson for a story, which confused the old man.  He said he had none.  He was baffled by Maniac, who was on his own, didn't go to school, and used to live with a black family.  At the same time, he was also quite fond of the boy. 

In time, Maniac gives up his plan to avoid parents and becomes quite close to Grayson.  The old man is his new family.  Maniac has a big heart, and cannot avoid attachment for long.  He and Grayson get along swimmingly.  He even teaches Grayson how to read.  

Unfortunately, this relationship does not last either.  Around Christmastime, Grayson unexpectedly dies and leaves Maniac alone again.  Maniac decides that enough is enough, and returns to the Beales.  They are his real family, from now on.