I belive the composit material is quite water resistant, The problam is the hardware. From what I can see none of it is stainless steal (SS) accept the studs/nuts that are used with screws to attach the boxs. And over time It will rust, There are things you could do to keep it relatively dry, but water will get in.
If you decide to go for it. You should do the following.
Raise the drawers off the bed of the truck by at least 1/2".
Use silicone as glue on all joints when assembling.
Replace all screws with SS screws.
I don't know of any SS drawer glides that are rated for over 100lbs, but there may be somthing. This would be nessary to find becouse other wise you will be replaceing the glides every couple of years.
It would be possible to do a low teck drawer glide made of HMWP (High Molecular Weight Polyethylene)
SS latches.
You should ask maybe they can sell you some of these options.