My new cabinet drawers shipped with slightly longer guides, so I removed the old guides and found the left side of the plywood carcase was secured only by the screws that attach the guides. Your trailer may well be different. I just took mine apart, so I know what I found.
The new drawers I received today are constructed of stapled and glued MDF with oak stiles and a veneer panel in the center. Many, many drawers are made with MDF, but it's not the ideal choice for use around water because it can absorb moisture, swell and disintegrate. If it gets wet. My suggestion for anyone concerned about that was to paint the raw MDF. Painted wood outlasts unpainted wood significantly. Not a really big deal but my first set got water damaged and discarded.
Some tips on building MDF drawers are here:
Can You Make Drawers Out of MDF? | eHow