We had similar issues with thin steel sheet bottom and rust and dirt accumulated.. Fortunately, the sheet metal was able to be saved...
Our renovation process consisted of:
1. Full cleanout with soap and water and soap pads/steel wool
2. Additional rust removal efforts using emery cloth and steel wool to remove rust mechanically
3. We drilled a couple small "1/8" holes in sheet metal at low points where water collected, to enhance drainage. Our spring and bumper pads were OK and didn't need replacement.
4. We then masked everything off and sprayed a Rustoleum primer and followed with light gray Rustoleum gloss paint for entire compartment, including sides of frame rails.. Painted only steel surfaces, not the aluminum trap door. Inside of bumper did get painted. Makes it much easier to find adapters and things in the compartment, and no rust to wear at sewer hoses.
So far, it has remained relatively clean and dry, and paint has held up
If metal were worse, we might have tried POR-15, which would permanently halt further weakening. Fortunately we did not have to consider replacement.
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...