black tank

Have you ever had the pleasure of using a pit toilet----outhouse?

If you notice all the solids are piled up and the liquids are off to the sides.

Leave your black tank drain valve open and you will get the same effect.
PLUS you are inviting sewer critters to come set up home inside your waste tank.
Sewer flies LOVE to build colonies and then you have not only a 'poop pyramid' but nasty flies to get rid of.

Think of it this way.....
A high level fast flowing river can move boulders but a small trickling creek can't even budge a pebble.
You want the fast flowing river ----dump tank when full/use high liquid to solids ratio.

Drain pipes in your home are rigid and do not normally move. When installed they are set to a proper "slope" which is best to make sure the water does not "out run" the solids and the reason they are "open" all the time. Additionally, in a home drain system normally people tend to use more water.

In an RV situation water conservation is normally of high priority so less water is used. In an RV situation the drain is normally anything but with a proper slope from the holding tank to the hole in the ground. Additionally, an RV does not dump directly from point of use into a small pipe or hose. It first dumps into a large tank and then into a smaller pipe or hose.

Bottom line as others have mentioned water flow is used to move solids. When the liquid "out runs" the solids you eventually have "backup".

THANKS FOR YOUR REPLIES.

Technically, the issue is that the black tank system relies on the fluids to carry the solids away in a rush when the tank is full and the valve is opened. If the valve stays open all the time, there is not enough liquids flowing fast enough to carry the solids away and they will build up in a pyramid-shaped pile directly beneath the toilet drop tube. This quickly dries into a horrible stenchy mass that will tax your best efforts to dislodge it. Water is your friend with an RV toilet. It really helps to fill the toilet bowl (by half-activating the flush lever/pedal) and then flushing that down after each use of the toilet. This will help break up the just-deposited waste and make it more able to be washed away when the dumping occurs. It's best to always wait until you have at least half a tank of black water before dumping as the volume of water is what helps the solids to be flushed out of the tank. When out with our RV, my schedule is dump grey water each day, the black tank every 3rd day. I'm assuming you know the drill to dump the black tank first then the grey tank to flush the worst stuff from the fittings and hose.

Trust us, the first time you try it will be your last.

Because all the liquids will drain away, but not the solids. You'll soon be left with a "poop pyramid", which will be stinky and hard to get rid of.