My cat is an out side cat he keeps going under my mobilehome and I keep fixing it and boarding it back up but he just finds another way in how do I stop him what do I do?

Put full skirting under the MH. Where do you expect the cat to sleep when it is zero?

What do you mean, outside cat? Do you let him in when he wants, I hope?
I made my cat a house out of foam insulation board, hoping she would use it if a fox or coyote or fisher chased her, and put it right by my door. I made the opening just big enough for her to squeeze in and practiced in the house by putting her food in it, and she went in and out of it for weeks, but she won't use it outside...
She hides under my car, so I park as close to the house as I can.
I hang Christmas bells from my door and taught her to ring them to let me know when she wants to come in. Took about a day and a half to teach her.

Maybe that's where its sleeps or it's a hiding place.