He may simply be looking into the tank to see how clean it is. That's what I do every time. I also use the toilet pedal to fill the tank up before opening the dump valve. He could also have his display monitor panel in the bathroom like we do. If our monitor doesn't read empty, that also means it's not clean enough. I would say these are pretty much impossible to do without getting into the bathroom.
I fill our tank up, dump, use our OEM in-tank rinser about 5 minutes, re-fill the tank and dump again. I repeat one or two more times and get the tank spotless every time now. LOTS of water works wonders. A to-do project on my list is to add a shut-off valve behind the toilet so I can look down into the tank without having to go outside to shut the CG faucet off. When we head home, the tank is fully empty and fully clean and ready for the next trip out.
Opening the slide a bit wouldn't be a problem but not sure if on a slope.