Clogged shower drain
Our shower was not draining very quickly. There is a small drain cover that can, sometimes, get filled up with hair and cause the water not to drain. We had taken this off and even shined a flashlight down the drain pipe and could not see anything.
We went so far as to bring in a garden hose with a spray nozzle and attempt to force the clog wherever it was, down into the holding tank. Still nothing. So, time to dig in.
We opened up the storage compartment, the same area that has the furnace and so much wiring.
Following the shower drain pipe down, we can see that it goes into a white component that says "waterless trap".
When we removed the trap, there was a big hair clog in there. Problem solved!
Here's the YouTube video explaining what we did.