Power systems in remote highway service areas face unique voltage stability challenges due to their distinctive load composition, characterized by a high penetration of constant-power loads such as electric vehicle chargers. This paper presents a systematic investigation of voltage-power coupling in such systems through analytical modeling and theoretical derivation. A comprehensive mathematical framework is developed, incorporating a second-order synchronous generator model, realistic line impedance parameters, and a mixed-load model with 7:2:1 proportion of constant-power, constant-current, and constant-impedance loads. The study first analyzes the individual frequency-voltage coupling characteristics of each fundamental load type, then examines their combined effects in the mixed-load scenario. Through rigorous derivation of voltage-power sensitivity coefficients, the research reveals that constant-power loads dominate the coupling dynamics, with both active and reactive power significantly influencing voltage stability. A key contribution is the establishment of an optimal power compensation relationship for voltage regulation, which is shown to depend solely on line impedance characteristics. The findings provide fundamental insights into voltage stability mechanisms in mixed-load systems and offer practical guidelines for integrating distributed energy resources in remote service areas.