A large-scale flexible service system with two large server pools and two types of customers is considered. Servers in pool 1 can only serve type 1 customers, while server in pool 2 are flexible -they can serve both types 1 and 2. (This is a so-called "N-system.") The customer arrival processes are Poisson and customer service requirements are independent exponentially distributed. The service rate of a customer depends both on its type and the pool where it is served. A priority service discipline, where type 2 has priority in pool 2, and type 1 prefers pool 1, is considered. We consider the Halfin-Whitt asymptotic regime, where the arrival rate of customers and the number of servers in each pool increase to infinity in proportion to a scaling parameter n, while the overall system capacity exceeds its load by O( √ n). For this system we prove tightness of diffusion-scaled stationary distributions. Our approach relies on a single common Lyapunov function G (n) (x), depending on parameter n and defined on the entire state space as a functional of the drift-based fluid limits (DFL). Specifically, G (n) (x) = ∞ 0 g(y (n) (t))dt, where y (n) (·) is the DFL starting at x, and g(·) is a "distance" to the origin. (g(·) is same for all n). The key part of the analysis is the study of the (first and second) derivatives of the DFLs and function G (n) (x). The approach, as well as many parts of the analysis, are quite generic and may be of independent interest.