The belly pan is coroplast (plastic cardboard LOL)
You can cut a window as described.....
But due to black tank leak I would remove the panel (panel can be reused)
The insulation on panel or in/around tank will need to be replaced......ain't going to remove the stink.
After removing panel...locate leak (drain valve/flange/seals, tank fittings, tank wall etc.)
After identifying leak.....fix/repair/replace
New insulation and reinstall panel
Good luck......old clothes, gloves, mask, water hose and a really good friend to help