I have not owned any pickups since 1974, but ave just purchased a new one. I am installing a back flip hard tonneau as well as a “bed rug”. I’ve also ordered a seal for the tailgate. This should make it pretty well water and dust resistant. I’ll be carrying shooting gear including shotguns etc. so I’d like to keep it as dry as possible.
There are seams in the truck bed that are somewhat open, in some cases the ground is visible. I’m thinking of caulking these seams to minimize water spray intrusion. The caulk could always be pulled off if it creates a problem. If water does get in, I can always leave the tonneau open until it dries.
I’ve had experience with those in trucks owned by others. For my purposes they are not so good. Ever crawl around on one with short pants? Or after it’s been sitting in the sun? Not so nice. The bed rug is cushioned and the surface won’t scratch any gun finishes.
I did consider that the spray on liners do seal all the seams, so I did caulk the seams on mine about an hour ago. I can always remove the caulk if I need to do so.
If you need a sealed storage area, why didn’t you just get an SUV and leave out the 3rd row seating? Would be a lot easier and ensure a dry storage area for your valuable weaponry.
I’ve never really seen water splashing up enough to worry about, but if enough gets in to make a puddle, it can take forever to drain out. I would drill drain holes and close them off with hole plugs so they’d be there if needed.