The Flory-Huggins interaction is measured for a model rod-coil block copolymer system, poly(alkoxyphenylenevinylene-b-isoprene), by fitting the interfacial segregation of block copolymer to a homopolymer interface and by using the random phase approximation (RPA) for block copolymers. The measured interfacial segregation of a block copolymer to the interface between homopolymers, fit with a self-consistent field theory (SCFT) simulation using chi as a variable parameter, gives a functional form chi = 34.8/T - 0.091. When RPA is applied to neutron scattering curves for the rod-coil system above the order-disorder transition, the theoretical structure factors are inconsistent with observed scattering curves clue to complex aggregated structures formed in the nematic and isotropic states. Using the Flory-Huggins parameter and a previously measured value of the Maier-Saupe parameter, the PPV-b-PI phase diagram may be converted from system-specific variables to dimensionless parameters. Under the assumptions that the rods are ideal nematogens, interaction strengths are composition-independent, and rod-coil and rod-rod interactions are local, this yields the first quantitative universal phase diagram for rod-coil block copolymers.