Heck with careful grind, it's a piece of scrap wood. Go at it with a 4 inch. angle grinder and grind the head off. Once you can pop the wood off and get it out of there then grind the old fasteners flush with the frame if screws or grind them short and us a punch to drive them out
Mine had screws to the frame and fiberglass tabbing to the trailer walls. Plus glue between it and frame around the well in the center. which I think acted as a seal and filler to help level over welds etc.
When you replace that wood if you have a front window I suggest coating that wood top and bottom and all edges with FG resin. Plus inside any holes for gas lines or wires. Between the window, the door hinges, the pigtail wiring you can be sure that sooner or later water will get in, especially if driving 55 mph in the rain. Making that board extremely water proof is a good thing.
I think I just found the last little leak in the front end. Someone put rivets and caulk in the weep holes, so track was leaking from the window center seam. Arrgh. Did the door seals and hinges, rivets with new caps and butyl, did the belly band FG seam where a bubble leaked, pulled the window and replace butyl tape. and still was getting a little wet spot in the same location. Trailer was parked so all leaks
went to the same low spot.
I expect your door opening may have shifted, there is a small piece of wood that the bottom of the door opening attaches to. When that rots the door tends to "spread" at the bottom. I'll dig out some pictures and post them.