Interferometric determination of thermal expansion and of normalized thermo-optic coefficients of ${\mathrm{RbTiOPO}}_4$ at four laser wavelengths are performed as a function of temperature. A suitable vectorial formalism applied to obtained data allows the establishment of the temperature dependence of refractive indices, and subsequent theoretical analysis enables one to predict that an extremum in the evolution of the phase-matching direction in the $(X,Y)$ plane should occur near $100°\mathrm{C}$ for type II second harmonic generation of Nd:YAG lasers, with a temperature bandwidth that can be as large as $117°\mathrm{C}$ for a crystal of $10\mathrm{mm}$ in length. Such unusual behavior is observed experimentally by recording the conversion efficiency from $20°\mathrm{C}$ up to $220°\mathrm{C}$ for various propagation angles of light in the $(X,Y)$ plane. Slight quadratic temperature dependence of the effective nonlinear coefficient is also observed.