The corrugated core sandwich structure is a promising candidate for the Integrated Thermal Protection System (ITPS) of Reusable Launch Vehicles (RLVs) due to its multi-functionality of load-bearing and thermal insulation capabilities. It is incorporated on the exterior of RLVs to protect the underlying structure from the severe aerodynamic and thermal loads during ascent to re-entry. ITPS can be exposed to drastically varying thermal loads during flight operations depending on its position on the vehicle, resulting in spatially varying thickness-wise temperature-dependent properties and thermal stresses. Thermomechanical analysis of RLVs components such as wing, fuselage, etc., along with actual geometry of ITPS panel makes it computationally infeasible due to their disparate length scales. This issue can be addressed by idealizing the ITPS panel as a homogeneous thick plate. In this work, a homogenization method is proposed to model the ITPS panel as a homogeneous plate to perform thermomechanical analysis. The methodology has been developed based on First Order Shear and Normal Deformation Theory (FSNDT) to incorporate the effect of out-of-plane normal strain in addition to transverse shear in the homogeneous plate model. The finite element computation of homogenized stiffness properties and sectional thermal forces and moments are presented in this paper. The efficacy of this method is verified with the comparison of the plate model and full-scale model of an ITPS panel with a thickness-wise temperature field. The comparison shows that derived plate representation is capable to encapsulate the response of the full-scale model efficiently.