OK, I thought long and hard about posting my recent experience but decided I may get answers to my questions as well as helping others to avoid this happening to them.
On the morning of my last day of the shakedown trip with our 07 DSDP, I decided to drain and clean out my black and grey tanks while hooked up at the RV park I had spent the night in.
My plan was to open the black tank valve, emptying it's contents and then flush with the grey tank contents. So, I open the black tank and....nothing??? Now I know there should be at least a third to a half a tank of contents. I was purposefully using a lot of water during flushing, knowing I wanted it to drain well. So I close the valve and open the grey tank momentarily and plenty of water comes out. So I open the black again....and nothing.
Background information - The previous owner had a SaniCon system hooked up. The first time I used it, the Black drained very, very slowly, indicating to me a possible partial blockage. But the grey drained as well as expected, since everything was slowing down to exit through the macerator and a small 1" hose anyway. On top of that, the way the 3" hose was connected to the RV valve assembly, it left a large amount of liquid in the hose to the macerator pump. It was a mess to clean up.
In my frustration, I removed the SaniCon system (it had broken mounts anyway) and went back to a standard drain hose. After repeated flushing, large chunks of prehistoric, rock-hard poo flowed out of the black tank and pipes. The black tank drained just fine after that. I suspect the slow draining over time through the SaniCon never allowed a good hard flushing of the tank and pipes, leaving behind solids that later petrified.
So back to my recent emptying....I am suspecting I may have dislodged another chuck that has blocked the pipe. So I connect a hose to the tank spray inlet, leave the black valve open and turn on the water flow. I can hear it spraying but nothing is draining. So I decide I need a larger volume of water in the tank to hopefully force out what is there. I close the valve for a couple of minutes allowing the sprayer to add water. I then open the valve....nothing. I close the valve and let more water flow in through the sprayer.
I decided to walk in and check the toilet. As I look into the toilet I see water bubbling in the bowl. So I decide to step on the foot peddle and open the valve.
Now I know some of you are yelling, "NO, DON'T DO IT!" Unfortunately, I did not hear you.
A geyser of brown water sprays up and out of the toilet. I ran screaming from the bathroom, outside, and shut off the water flow. I then opened the black valve again and ...whoosh...everything dumps and empties.
So, my probably stoopid question is...how and why was there an apparent vacuum in the tank that seemed to allow everything to dump as soon as I broke the seal at the toilet? Why wouldn't the roof vent keep a vacuum from happening? Or am I barking up the wrong tree?
Oh, after about an hour of cleaning with clorox and a couple of roles of paper towels, the bathroom is spotless once again.
HoboBob