I’m assuming that you mean the long grey plastic fitting running from the front doors to the rear bumper on either side?
There are screws that hold the sides to brackets onto the chassis about half way down; screws just under the front corners; screws under the corner pieces. You can then lift up the entire panel like a trap door and unhook it from the aluminium trim that runs along the ‘waist’ of the vehicle.
Oh, I should have mentioned that on the habitation door side, there are extra screws/brackets and an aluminium trim round the bottom part of the doorway to remove. Also, this piece is very long, BUT has a thin, weak point half way along (where the doorway and step cutouts are). I would recommend either rigidly supporting this weak, bendy centre section (piece of wood and gaffer tape, maybe?) or have three people handling the removal.
I did an inspection of the metalwork on mine a few years ago.
I just undid the screws along the bottom as Barry said, then lifted the grey panels which remained “hinged” along the top edge and propped them while I cleaned and waxoyled.
It might be due another coat of looking at, when I get the urge.