Thank you for your reply.
When he pulled the valve handle this morning it didn’t open the valve to flush. The weird thing is, it’s BOTH black tanks (the model we are in has 1.5 baths). We have been living in our unit, full-time, for 3 years.
Ok "didn't open" could mean the handle pulls easily, as in no longer connected to the valve, or it could mean won't pull because it's stuck. Or perhaps they pull normally but nothing comes out the sewer outlet.
If they pull like they're disconnected from the valve, you'll need to inspect the cable connections at both ends. Certainly it's puzzling that both would fail at the same time. Nevertheless, you'll need to take down the rear wall of the pass through basement storage to inspect the point where the cable attaches to the handle assembly. To get to the valve end, you'll need to at minimum remove coroplast mounting screws from the front section, in the corner near the sewer outlet. Or you could try to find a mobile servicer to do that for you.
If they pull normally but nothing comes out of either, there might be something blocking the sewer outlet. When the ground is cold enough, if you have a slow leak, you can build up an ice dam that blocks everything. Put a bucket under the sewer outlet and remove the hose to see if there's a blockage. It's unlikely, but it's also possible something is in the 3" pipe between the point where the 2 black tank pipes come together, and the outlet.