Our 2002 Thomas had a bunch of rust in that area. After cleaning that up, there was still a space in the lower front door (but not enough to get a cat through). I used some lumber and a pipe-clamp. I put the lumber on the first stair up from the bottom such that it protruded about an inch or so past the edge of the opening in the bottom stair where the door is SUPPOSED to close against. Then I closed the doors, put a board on the outside of the door along the bottom, and used the pipe clamp hooked at the back of the bottom stair and outside of the door "board" right at the end of the front door for most leverage. I slowly cranked the pipe-clamp to bend the door. I also moved the "stairs" board up (by piling up about 8 inches of lumber) to get a little more bend because there was need in that area as well. Take it slow and easy and check for fit, removing the "stairs" board so the door can close.
Darn ... I knew I should have taken pictures!
Edit: Note that the "stairs" board should NOT be placed above the solid portion of the door unless you want a cracked window.