Safety code (NFPA 52, Texas Code shown here) require "All pressure relief devices and pressure-carrying components installed within a closed compartment shall be vented to the outside of the vehicle in a suitable location." This does not apply to CNG components under the truck hood and in the truck bed - until you cover the truck bed with a tonneau cover or a camper shell.
So, if a truck bed storage system is used that closes in the bed, including the CNG tank, then a venting system must be installed first. Note that the Honda Civic, which has the CNG tank in the trunk, has a venting system installed at the factory.
The second consideration is that most of the truck bed mounted CNG tanks extend above the side of the truck bed sides which makes a tonneau cover nearly impossible.
Weather Guard Truck Drawer
An easy solution I've seen is the use of a WeatherGuard steel storage box with a drawer. This avoids the whole CNG tank venting issue by leaving the bed open and keeps the contents dry, secure, and easily accessible via the large pullout drawer. The box can hold up to 425 lb of weight on top of it. Downside is that it would be difficult to haul a refrigerator in the truck (but that's impossible with a hard shell tonneau cover or camper shell also).