( This has indeed come up before so I copied the answer from this post to save myself the extra work )
Before Sam hit the road with Dean, their family lived with an outhouse. It was placed under the motel, as deep as they could go to where they were able to dig holes. This was also used by Walt to dispose of the remains of the rats he would kill and skin. When full they would cover up the hole and find a different place to dig.
Afterwards, it became easier. Sam can simply use the sink in the bathroom. He’s able to get himself around the entire motel room without any help, and he doesn’t have to worry about being grabbed if he’s out in the open. For his teeth he can snitch some toothpaste from Dean while he’s up there. There’s only been one time Dean almost walked in on Sam while he was using the sink, and the scolding Dean got sent him almost running.