Black tank flush overfill

My guess is that you have a loose fitting vent pipe coming out of the top of your black tank and that is where the water that was in your underbelly was coming from. If that is the case there probably isn't anything you really need to "fix"

Actually, you need to fix how you flush the black tank. Things can go very wrong when attention to detail is lost. When flushing, if anything distracts you, turn off the flush line. You got lucky once. The next time could get very ugly.

I would not say OP was lucky yet? Other than out the toilet, or out the roof vent, there is no place else to go. Something is damaged, likely the black tank. You will need to gain access to see where the problem is, by cutting a window, or removing a portion of the enclosed covering. If you have fiber insulation, that is wet, that will need to be removed/replaced.

Jerry

My guess is your black tank has a "slip fit" on the top with gasket..

HOPEFULLY - You just need to drop the underbelly and allow EVERYTHING to completely dry out.. Possibly replace insulation if there is any... Once insulation is removed, give the underbelly a good spray of disinfectant before closing up.
This happened to me but it was not from a hose left turned on.. I Used to ( ROOKIE MISTAKE ) keep my black tank flush hose hooked to the trailer with the shut off valve closed the night before leaving. my valve failed and slowly filled the tank to the point of overfilling and spillage into the underbelly.

You just filled your RV with SEWAGE. Once the tank is full it fills the vent pipe and once that is full it boils over the top of the pipe to run back down the outside of the pipe and out the underbelly.

Look on the roof your vent pipe is not sealed it just vents under the rooftop vent. Your tank fitting did NOT leak. If it was not tight you would have POO JUICE constantly weeping traveling down the bumpy roads with POO in the tank.

To avoid this EXACT problem as it is VERY easy to get distracted I set the timer on my phone for 10 minutes with a LOUD Dog Bark when I close my tank valves and hav the flush system activated.

Just wait for the SMELL once the heat cooks the POO. If this happens you may want to file an Insurance claim.

Step away like a boss. Nothing happened.

You are lucky it did not rupture the seam on the black tank.

Ken

One thing to do is get a mechanical Lawn sprinkler timer. THAT will help stop this from happening.
Also, this helps when people use a Y hose connection with shut offs. IF the cutoff seeps/leaks, then it will slowly fill the black tank. Doug

https://www.amazon.com/Orbit-62034-Mechanical-Watering-Timer/dp/B006FLGHGE/ref=asc_df_B006FLGHGE/?tag=hyprod-20&linkCode=df0&hvadid=216531775325&hvpos=&hvnetw=g&hvrand=979541683741792855&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027230&hvtargid=pla-350187149251&th=1

The type Y

https://www.amazon.com/Splitter-Shut-Off-Irrigation-Watering-Suitable/dp/B07SZMFXPH/ref=asc_df_B07SZMFXPH/?tag=hyprod-20&linkCode=df0&hvadid=343238573411&hvpos=&hvnetw=g&hvrand=6200864078994098471&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027230&hvtargid=pla-966760518432&psc=1&tag=&ref=&adgrpid=71762478951&hvpone=&hvptwo=&hvadid=343238573411&hvpos=&hvnetw=g&hvrand=6200864078994098471&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027230&hvtargid=pla-966760518432

OK, one thing that can happen when you do this, it can BREAK the 3 inch down tube from the black tank. The Pressure involved when you do this is tremendous. 45 psi or more has filled the black tank and now the tank has expanded(swollen) and popped either the 3 inch down tube loose or the 1.5 inch vent tube loose. We know this because instead of water coming out the roof vent and overflowing on the roof it came out the bottom from the top of the tank. Because the tank broke open at the tank which is a good thing instead of out the roof vent. NOW, if you do NOT fix the problem on top the tank, you will have black tank odor migrating into the lower bottom cavity because it is NOT sealed now. The RVIA standard for both Gray and Black is to hold water when overfilled. With Black we fill until the water is just below the toilet ball valve and on Gray with water coming into the shower or tub. Also, unless you access and clean out the top of the tank and the underbelly and insulation, you will now have a musty smell all the time. Both inside and out. Doug

Why do people not leave their black tank valve open when flushing the tank? Valve open, tank won't overfill, why would I want to risk a disaster?