Coexistence curves of square-well fluids with variable interaction width and of the restricted primitive model for ionic solutions have been investigated by means of grand canonical Monte Carlo simulations aided by histogram reweighting and multicanonical sampling techniques. It is demonstrated that this approach results in efficient data collection. The shape of the coexistence curve of the square-well fluid with short potential range is nearly cubic. In contrast, for a system with a longer potential range, the coexistence curve closely resembles a parabola, except near the critical point. The critical compressibility factor for the square-well fluids increases with increasing range. The critical behavior of the restricted primitive model was found to be consistent with the Ising universality class. The critical temperature was obtained as T-c=0.0490+/-0.0003 and the critical density rho(c)=0.070+/-0.005, both in reduced units. The critical temperature estimate is consistent with the recent calculation of Caillol et al. [J. Chem. Phys. 107, 1565 (1997)] on a hypersphere, while the critical density is slightly lower. Other previous simulations have overestimated the critical temperature of this ionic fluid due to their failure to account for finite-size effects in the critical region. The critical compressibility factor (Z(c)=P-c/rho(c)T(c)) for the ionic fluid was obtained as Z(c)=0.024+/-0.004, an order of magnitude lower than for nonionic fluids.