Woke up this morning and immediately tested the slides...and they are working. The cold mornings for the last month have meant non-operation but today the slides are fine.
So, I'm calling it a poor ground connection on the ground stud at the hydrualic manifold. There's 7 ground wires with ring terminals on the stud and they all appear to be in good condition, but for the slides to go from non-functional to functional just after reefing on that nut, and for it to stay functional kind of points to a poor connection there.
When I can get an impact tool to remove the nut, I'll take that stack of grounds off, clean and tug test them all, and add some corrosion inhibitor.
So that's it, a bad ground. Thanks for everyone's advice.