The interior temperature T_i of a fluid undergoing thermal convection at high Rayleigh numbers reflects the heat flux through both boundaries, and thus the relative thickness of thermal boundary layers. For constant-viscosity convection, T_i is the mean of the boundary temperatures. If viscosity \mu decreases with increasing temperature T, however, convection occurs beneath a stagnant layer and T_i increases relative to the isoviscous case. We show that the predictions of T_i by boundary layer models for both small and large viscosity variations agree with published experimental data in the appropriate limits. Of particular interest to applications involving asymptotically large viscosity variations, is the result that the temperature difference across the hot thermal boundary layer is proportional to the rheological temperature scale -1/(d log \mu/dT). Experimental data indicate that T_i does not depend on the Rayleigh number or Reynolds number, at least for Re < O(100).