Soft floor discussion (pics and steps to fix included)

I had the same problem. I removed the whole floor back to the frame, then replaced it with 2x6s that rested 1/2" on the frame and were screwed to the aluminum frame under the door. I covered top and bottom with 1/4" AC exterior plywood, then lagged the steps into the 2x6s and bolted to the frame. Has worked well for 2 years, hopefully much longer.
Good luck.