I have been full time for over two years and this situation has perplexed me.  

I empty and flush the 25 gallon black tank weekly.  Usual procedure is to pull the black valve, let the tank drain, then turn on the flush water.  Using the timer on my my watch set for five minutes, I close the valve and reopen after that time.  A few flushes and the tank will read empty.  Put a few gallons in the tank, then some treatment and it is job done.

Tonight I deviated from the usual procedure.  I noted the tank read as full, but yesterday it was at 1/3.  I decided that was a spurious reading and decided to run the fresh water for two minutes, then open the valve.  

At a little more than two minutes I caught a whiff of sewer, then opened the valve.  I was surprised at the flow volume coming out of the pipe, and horrified to have brown water coming out of the roof vent!  Chernobyl!

I kept the water running and the valve open, then went into the RV to inspect for damage. The toilet bowl was full of backup, but thank God no overflow.

Once the situation was stable I flew my drone over the roof.  No obvious blow out but a puddle of brown water in the corner.

I am at a loss to explain WTF happened.  Any ideas?

Do you ever move the unit enough to slosh stuff around in the tank?

Been parked for a few months.  I will dump a bag of icecubes down the toilet and drive around.

A black tank cleaning wand gets it empty and clean every time.

Leaning that way.  Still trying to discern WTF happened.  There is no way that tank was full.

you are very lucky it did not come up through the toilet in the RV.

It sounds to me like a ball of TP/ solid waste clogged your drain.  When you turned on your flush it had to go somewhere.  So it went up the vent that allows the tank to drain without causing a vacuum.

Again, you were very lucky

What doesn't go down,

Comes back up,

spinning bowl, water goes around,

Overflows at the drop of a hat,

Makes the women scream and gets the men mad.

Plugged drain. Shit has to go somewhere.

When I bought my TT, the dealer said to always leave the valve open when running the flush.

My material says same thing. If you just want more water through it get a wand and run it down through the toilet .  I think you are forcing more water than the system is designed for when using the flush and keeping the valve closed. seems like you sent some goo up the the vent.   Not sure but you might want to run some water down the vent ?  Not sure if that is a solution or not.

That has been my practice as well.  My intent was to run a few gallons through the sprayer, then pull the plug as a sort of experiment.  Turned into a FAFO sort of deal.  

There is NO WAY that tank was anywhere near full, but damned if I didn't have the brown geyser.

Yeah, cleaning out the vent has occurred to me.  I think I'll hire that done.

Final Update:
Aerial recon indicated a puddle of poo water in the left rear corner.  I was able to hose down the roof and deconaminate as best as I could.  

I flushed the black tank for 20 minutes, then closed the valve for 2 minutes.  Opened up, and repeated for 4 minutes, opened and then for five.  Water ran clear in all instances.  I then closed the valve and ran for two minutes to put water in the tank, then added Happy Camper treatment.

RV goes into a storage lot tomorrow while I move into my home.  Once the house is stable I'll have a professional shop assess WTF happened and give the vent a cleaning if that is required.  

Thanks, all!

John

Get on a RV website and look for "the pyramid of poo" if you have never heard of it. Seriously, that is what it is called. If the tank is not actually washed out with a wand, or driven down the road and sloshed around stuff will pile up and harden into the dreaded "P.O.P.". It can be very hard, or I guess even impossible at times, to fix.

I've never had that problem.  Been poo'ing in this camper for 5 years now.

1. Use correct TP
2. Use correct amount of additive. Most of the time I add a tablet each morning.  Overkill?  Maybe but I don't have a PoP!
3. Add a decent amount of water to the bowl, poo, then flush.  
4. Quick dump at the dump station won't cut it.  Let that tank flush run good and hard. (with the valve open)  
5  Before you put your TT up in storage for the off season, that last trip, really hit is with treatment and a good flush when you dump.

Me neither, and no one will if they take care of the tanks correctly. It'd a lot easier to avoid than fix.

I worked hard to avoid the pyramid....I'm pretty sure that isn't what happened here but it is so anamalous that I can't rule it out.  

RV goes into storage tomorrow.  I am going to put 5 gallons in the tank along with a big dose of Happy Camper and let it cook in the Alabama sun for a few weeks, then give it a good flushing.

The other mistake I have seen people make is when at a full hookup site, they leave the black tank valve open.  

Black tanks need water to work.

Yup.  It ain't the home sewer system!

You can clean the vent with a garden hose and old twist spray nozzle.
Just lower the hose by dropping it into the vent down to the tank. The sooner the better, if you let the debris dry fully its going to set up.  


The tank had to be either full or over pressurized to push fluid out of the vent.
Since the vent was obviously working it wasn't over pressurized, that points to the full reading being correct.
I'd test the level indicator, the overflow points to the 1/3 reading was the erroneous reading.

Cleaned and flushed weekly, only me as an occupant.  Your theory and proposed course of action. makes sense.

Despite my best efforts I think I'm looking at the dreaded "mountain" situation.  The advice above about having the vent inspected/cleaned is very sound.