Camping stores carry a product called Sturgi flow or Extend a flow. That can be placed in line on the tank after the regulator that provides a t connection. Some have hoses included. However as the previous poster mentioned, be sure you understand the diff between hi and low pressure regulators and which your grille requires.
In the case of the FR3 if you were to do this, unless you want to cook on the street /drivers side you would have to either run the hose under the coach or around the front.
Another option is to have a qualified gas tech tap into the line that runs to the kitchen stove and heater. There is a diagram of the gas line on this forum in another thread called "FR3 diagrams".
But the question was "what's the easiest and cheapest way" way.
I buy 4 mini green cylinders from Walmart on sale. But I'm not full time camping.