The RFSVslide out document box has a rubber seal on it, some are a better seal than others.
One of mine is water tight, I've had it fully under water for a while with no leaks while another one of the same design leaked a fair amount.
The boxes are riveted to the slides, then the slides have a retaining bolt or two to stop them coming out off the slides.
The rear compartments that are located behind the rear wheels are a proper sort of compartment with a floor in them, but they are not really waterproof at all.
If no one else puts up pics before hand, I'll post some pics for you in the morning.