Truck vault is a great product. Have had one since 2018.
Pros:
Secure (I have the Kaba Simplex combination locks - these are the same as I've seen used in certain military applications.
The locks were easy to change the combination.
Cons:
Vehicle specific. The "body" of most are ~48"x64", meaning it will fit between the wheel-wells of most short-bed trucks. You can order them for long-beds as well. There are "wings" that come off the sides of the body to create a completely flat deck. The wings have magnetic latches that hold them in place, yet still allow for some unsecure, yet out of sight, storage to the sides of the box. If you change to another make/model vehicle, or possibly even another year model of the same vehicle, the wings will no longer work as the interior of beds change. Mine does not currently have the wings attached, but my current truck has a camper shell with no windows.
Not waterproof. The standard Truck Vaut is not waterproof and is designed to be mounted inside an enclosed location. For a pick-up, that means either a tonneau cover or camper shell. They do make an Extreme Series All-Weather TruckVault, which they say under most circumstances is waterproof. My last truck had a camper shell with dark tinted windows and it was very difficult to tell there was anything other than a flat bed inside the camper shell.
Takes up space between the top of the Truck Vault and the bottom of the tonneau cover/camper shell. Figure about 12", or the height of the fender wells.
Heavy. If you had to pull it out of the bed, it's a minimum two-person job. Also, it will beat into your payload. Maybe not much, but you need to figure out how much payload you have. The Truck Vaults weigh between 130-265 pounds.
I believe this is one of the best investments I have made for travelling. I have travelled from the MS Gulf coast to AZ, TX, PA, VA, SD and WY on vacations as well as semi-locally to LA, AL and FL. A suitcase for the wife and I, which we kept in the back seat of the truck, is all we took into the hotel room. Never even opened the camper shell except at matches.
There are two drawer heights - 5.625" and 10.125". I have the 5" drawers and have not had an issue carrying everything I need (firearms, ammunition and leather for two people for a 12-stage match, plus side matches and warmups. Our leather stays in the vault unless we are going on a trip somewhere that we won't be using the leather and would rather have extra secure storage.
I have also heard good things about the Decked system but I've never seen one in person to provide a comparison.
You can sometimes find like-new/demo models of various Truck Vaults on their outlet sight.