My 17 375 has the outdoor kitchen as well. We've been camping with the nite temps dipping into the upper 20's before, and the slide out, with no issues. However, IMO I think you have a good idea on closing that slide at nite. If you do take a look see inside to make sure the slide is not covering your heat vents to that area inside. Another option would be to insulate the water lines with the pool noodle looking insulation wraps.
2017 North Point 375BHFS
2015 Silverado 3500HD Crew Dually Dmax 4x4