He thinks that people are living under him (there is no way they can) he turns the gas off at the tank to keep them from using it, he says that they steal his flashlights and food. But he puts things up and can't remember where he puts it. What can I do?
If he is on meds you might want to let his doctor know in case his meds need adjusting.
Make sure he is taking on adequate fluids to ward off Dehydration.
What are your plans for dad for when you think he can no longer live alone?
Does he call the police about the person under the trailer? He might decide to or he might solicit the neighbors help. If you are unsure about next steps you could call your Area Agency on Aging and ask them to evaluate your dad for any services he might be eligible for and to do an assessment. Someone living under your house would be very stressful. I hope you can get him help.
My dad had delusions as a result of end stage liver disease. He thought people were out to get him. While I didn't subscribe to his fears I would tell him that I had taken care of it and that he wouldn't be bothered anymore. That's as specific as I got: I took care of it. I told him there was nothing to worry about anymore and that I would stay on top of it for him. He was relieved.