Black Tank Issue

I have been having a terrible time with our Black Tank for a year or so! When we are camping for a couple of days first when you press the pedal down to flush it has a blup & then it goes down and then every time you flush it gergels for a few minutes. After another day or so the waste is visible and I have to use my hose that is made to clear clogs. It buys me another couple days is all. I know it sounds like a Venting problem so I put a water hose down the Black Tank Vent on the roof a couple of feet, opened the dump valve with a clear piece on my dump hose and then turned the water on. Yep, it came out thru the hose, so to me that meant the vent was clear, right? So I figured it would be good now. Went camping last weekend & it was the same!

OK, smart guys, what is going on?

Bob