Our very first black and grey tank dump happened this past weekend. All was going well until Laurie unscrewed the cap to the tanks. Out spewed liquid! The levers were closed so we were flabbergasted as to what the liquid was and if we'd get in trouble with the campground maintenance guy, etc.
We pondered what we might have done. I did push the limit when the grey tank showed 4 lights (full) because I needed to wash the dishes and my hair! Was that the reason? Was it something else? I know we also need to get better at leveling both front to back and sideways!
Of course, we reached out to our expert RV guy, Jeff Johnston.
"Some RV companies fill the grey and black tanks at the factory as a leak-test for the system. They occasionally forget to drain them. When you took the pipe cap off to attach the hose, even with the valves closed, there's a bit of water between the cap and valve. It should have been fresh water from the factory.
I've run into the full tank problem numerous times when doing road tests on new rigs. I usually drain the tanks at home, just open the valves and let the clean water run off. On one test my Mom and Dad were with me and when Mom showered, it immediately started backing up into the shower stall. I had to go out -- we were in a relatively private, remote desert location -- and discreetly allow the grey tank to drain before she could finish. We were surrounded by heavy brush so the discreet part was easy.
If the tanks started empty, and there was some fluid present when the cap was removed, it could mean you have a leaky black or grey tank knife valve. Your nose would tell you which it was. I think this is the least likely scenario.
Those tank fill level lights are guidelines only. They are notoriously inaccurate, even on new rigs, but they're cheap to install so the industry keeps using them. If your rig is off-level a bit, that can cause the water in the semi-full tank to slosh towards the level indicators and give a false full reading. Opposite is true if the rig is tilted the other way. This is especially prevalent with broad, flat, shallow tanks such as the typical holding tank. In the black tank debris can also eventually build up and cause false readings."
Thank you Jeff!