View of how the plugs are fitted into the caravan wall.

View of the plugs in various positions of fitting.  Note the plug has been labelled to use No 6 screws.  The screws must not be longer than the thickness of the caravan wall.  A good guide would be to use screws slightly longer than the length of the plug.
Note the arrow in the bottom wall plug.  The left of the plug is drawn into the base of the plug causing it to balloon and holding tight the plug and screw at it's tightest position.
Don't over tighten as the wall plug will strip and loose all it's adhesion for the screw.