How do you remove stains from mobile home walls?

Walls are walls, wherever they are. - This problem depends on what the stain is and you haven't said.

Possibly easiest to simply repaint the wall - first with a 'stain blocker' such as Zinnser Bull's Eye or Kilz Stain Blocker Primer. -These products work well and ANY paint can be applied over them. -(if the wall is papered, then you should remove paper first.)