If you have a bulk tank rated for over 119 gallons, technically, it is supposed to be placarded. However, in a private vehicle there is little if any enforcement of that.
Commercial carriers are required to have placards and corresponding licenses if the tanks are rated at over 119 gallons, or any combination thereof aggregated to 1,000 gallons or more. This would include a pickup truck with permanently mounted 120 gallon or larger slip tank owned and operated by a commercial carrier.
The rub is if the tank is plumbed directly in to the vehicle's fuel system those placard requirements are eliminated as the vehicle is no longer a "bulk tank transport" vehicle. However...the tank must then meet all the requirements of a vehicle fuel storage tank, not a transfer/slip tank. So, plumbing a Weatherguard L tank into a trucks' fuel system is technically a no-no.
But, like I said, enforcement of this in a private vehicle rarely if ever occurs so as long as you're not leaking or driving like a maniac chances are you should be fine on your trip.