RV Repair: Toliet to black tank, rv manufacturer, vacuum breaker

Question
I have been RVing for 12 years and this is our third rig.  We have owned it for 2 years and have never been able to successfully dump the black tank.  We have had it serviced by three different RV dealers.  They have all told us it is fixed. Yet every time we go out the toliet below the bowl valve fills out and nothing comes out of the tank.  The tank has a back flush valve (outside) and I can fill the black water tank this way (and it dumps just fine).  This is so frustrating it is unbelievable.  I am suspecting that there is a problem between the toliet and the tank and intend to dismantle the system after it is cleaned out.  What should the down pipe look like going from the toliet to the tank?  Should it be a small extention  into the black tank or a long sloping pipe?  I have tried a hand auger (6 feet long) but that hasn't helped.  Any advice would be appreciated.  Thanks

Answer
Sometimes the vacuum breaker is defective. I have seen ones that the valve inside of it is backwards, or it was installed backwards. You should have an owners manual for your toilet and flush system. If not, your unit is only 2 years old, so call customer service at the RV manufacturer. Don't pay any more money. There is probably something that the factory techs can help you with. It could be something that is very common.
Dave