No black water tank venting

Well yet another deficiency in this 2006 Landmark. After 65 hrs. I thought I had them all- not so. We just returned from 2 weeks at lake-every time we flush the wc-we get a huge release of gas. It is violent enough to blow the WC bowl contents up and out-hence we do not flush without closing the lid. I went up to roof to see if I could open up what I thought would be a plugged roof vent. I found what appears to be a vent for the autowasher RI on the starboard side and two vents on port side-from bathroom group and kitchen sink. I fashioned a pipe cleaner out of a 8oz triangle lead weight that I bombarded down the three vents. I stopped when I retrieved water from each vent but no joy. The WC still flushed the same. I am at home for a couple weeks and then we are out again- I really would like to fix this. Does anyone know if they wet vented the black tank through the auto washer vent? I'm thinking the black water tank may not have been vented. On a second note-the pipe that was up to the roof vent looks suspiciously like plain old 1 1/4" polyethylene water pipe as opposed to required DWV spec. pipe. Any idea where I could find a shematic?
I am retired plumber and have had and built RV's since 1969.

Hoser43,

I'd suggest taking down the basement wall behind the UDC so you can see the top of the black tank. Perhaps you'll be able to spot the vent pipe and see where it goes.

This is a 2006 rig, if it was a plumbing defect, it should have been doing it since it was built. Did you just buy it used? It is possible that the vent pipe extends too far into the tank and gets blocked by water, thus not venting gases before the tank is actually full. Try dumping the tank and then adding maybe 20 gallons of water back into it. Then try flushing and see if it still "burps." What is an "autowasher RI?"

John, this is a new to him last fall trailer.

"Hoser" any chance maybe your vent pipe either broke, got dislodge, sunk into the tank after your incident last winter?