We study in this paper the design of the vehicleto-everything (V2X) network for vehicle platooning scenarios. We focus on the impact of packet relaying using Road Side Units (RSU) on the application-level performance, namely the inter-vehicle distance in the platoon. The RSU extends the coverage range of the platoon leader and allows handling the well-known Predicted Cooperative Adaptive Cruise Control (PCACC) scheme. We develop a Markov model for evaluating the performances of the different communication links (inter-vehicle and vehicle-to-RSU), using licensed or unlicensed spectrum. We then develop a cross-layer approach that adapts the application layer (PCACC control parameters) to the observed Medium Access Layer (MAC) performance. The simulations results prove that relaying allows to greatly reduce both the communication link failures and the intervehicular distances while introducing minimal delay in the system. Furthermore, even if using licensed spectrum for the relay link it drastically reduces the Packet Error Rate (PER) compared to the unlicensed spectrum case, the robustness of the application layer scheme makes the latter case viable for the considered scenario.