Well welcome to the forum, Dawn & Alex.
You have company your the third or forth owner of a Northern Star on forum the pass year.
You answered your question the slide being out has collected water or moisture on a Actuator
in the switch box for your slide lock and the contacts can become corroded or dirty and need to be cleaned up or the Actuator replaced.
What you can do is gently tap the box and maybe the contact will lose the rust or make enough contact to activate the rest of circuit for slide to move.
Slide lock problem will not allow the slide to move as you have found out.
A good contact spray cleaner
may clean up the contacts also and Newmar has the rubber seals that have been improved they say to keep out the moisture from entering.
I will give you links out of QT's # 3
for the problem and there are slide spec's in QT's 1 & 2
plus lock information.