Plumbing in a hydronic heater should not be an issue.
As you have mentioned, the prime focus here needs to be not affecting the cooling of the engine. To that end, you need to tap into one of the parallel heating circuits. The obvious one is the truck's cabin heater circuit.
The problem with fluid motion is that it will always take the path of least resistance, so with your heater being 12 feet away you may not get any flow, unless (as you say) you add a small circulation pump.
Alternately, you can plumb your heater in series with the cabin heater circuit. The cabin heater circuit itself (as mentioned) is a parallel circuit in the trucks cooling system. That is why you can turn off the coolant flow from the dash in that circuit and not blow up the truck. This is how I have plumbed in the calorifier (water heater) in my truck, but it is only about 4 feet away, not 12 feet.
Personally, if I were doing the install I would use a small circulation pump and plumb the heater into the cab heater circuit in parallel.
Where you tap the "to the heater" line into the circuit is also important. If done after the cab heater tap then the cab heater needs to be on for you to get flow to your additional heater. If done before the tap then you may have to include your own tap for that heater circuit, or you won't be able to turn the additional heater off.
If you were to use a small, positive displacement circulation pump it would give you good flow to your additional heater and when not running would act as a tap. If you wanted to use a separate tap and a normal centrifugal circulation pump, then you could use a solenoid based or motorized valve, that you could turn on or off from the cab. I use motorized valves in my hydronic setup to give me different flow options.
