problem solved...maybe...maybe not.
the vent cap came off easy without disturbing any caulking....loosen screw and pull off.....so i stuck a hose down there and let it run for 3 minutes....pulled the handle on the black water tank....zero flow...now puzzled, pulled the gray water handle and out came 20 gallons.
put more water down vent pipe...checked gauges inside....sure nuff gray water tank filling.
so vent pipe not connected to black tank....guess the black tank flush opening is the vent....that's why my cap comes off going down the road.
factory plan it this way...who knows...but i aint gonna worry bout it any more.
which makes me think that people that get nasty smells in the bathroom are plumbed the same way but their flush cap not blowing off like mine.
2008 F250 6.4 diesel with 50 gal aux tank towing a 2015 Rockwood 2109s Minilite.
2015 Miles and nights
2016 miles and nights
feb. 380/7......march 540/3