Jim, if you search this forum or others such as IRV2, you'll see this is a common issue.
Ours stopped working several years ago. We spent many hours manually tracing wiring, fuses, etc. Ultimately, we changed the rocker switch (in our case on the dash). It stopped working about a year ago. Tested the switch. Something else this time. Not as worried as we were the 1st time.
We are byway as opposed to highway folks. We have close to 20K miles during slide lock out of commission periods, Slide stayed put. Have read a number of like experiences.
Having said that, you are correct, the mechanism is enclosed in a plastic cover in your upper cabs. Cover easily removed. Dry paintbrush will remove all the dust.
Solenoid operated plunger. Lay a multi-meter across the contacts for voltage check while cycling the rocker switch. Perhaps a little dry lube on the plunger? Also, check to see if the drain tube is clogged. That will be a small orifice on the outside wall opposite the lock.
Good Luck!
Fair Winds and Following Seas