But Pope equalised just before the hour from the impressive Nicky Law's cross, before sub Gray was credited with the second on 75 minutes although it looked suspiciously like a McWilliams headed own goal.
The win was sealed two minutes from time as another trialist Andy Liddell crossed for Ellison to head in the third at the back post.
The Millers squad players who didn't feature last night will get a run out in tonight's friendly at Mickleover Sports, Station Road, 7pm kick off.
Team: Warrington, Lynch, Sharps, Fenton, Green, Law (Liddell 72), J Taylor, Mills (Harrison 66), Ellison, Warne (Gray 56), Pope.
Subs not used: Tonge, Joseph, R Taylor, Nicholas, Annerson.
Crowd: 758 (110 Millers).