question about black tank rinse

I don't have a fancy back filler. But I have ran a water hose into toilet and used a Flashlight to see when its almost FULL.
the sound of the water changes when its close to the neck

First, I leave the rinser running while the tank is draining. Then when the bulk of the contents have drained, I close the valve, leave the rinser running, and watch the black tank level indicator inside. When the full LED barely starts to blink, I hop back outside and open the valve. I only do this every few months (we're full-timers). In between, I use the "geo method" to clean the tanks on a travel day (dish soap, borax crystals, and about 1/4 tank of fresh water in each tank before hitting the road).

Rob

You'd be hard pressed to find the volume at a campground that's more that 10 GPM, so a couple of minutes would be safe.If in doubt, take that 5 gallon drywall mud bucket you have in the basement with you and time how olng it takes to fill that up. Last time I checked at my house, good pressure municipal water was 6 GPM.

I go one to two minutes. I hooked up a water flow meter a couple of times to test, and in about two minutes, I get 4-5 gals. Initially I might just go a minute, but after a couple of times, I'll go for two mins or maybe even 2.5 mins. Of course, when Im doing three or four or more flushes, it's at a FHU site, or a dump station with no one in line.

Mike

I never go more than 30 seconds, mainly because there's this big warning sign on my coach.