A couple of thoughts come to mind.
The vent pipe was supposed to be set into the tank, barely protruding into the tank. It has happened that the pipe slips down into the tank, although 1/2 way seems a bit extreme. If that happened, when the tank filled 1/2 full, the vent pipe would be submerged and unable to vent the fumes.
The second thought. This one happened to me. The stink was overwhelming when we drove. I checked the roof vent. It was O.K. I was baffled. Then one day I was searching around for something else, and I found an interior vent hidden behind my furnace, and... you guessed it.... it was not working. I replaced that cheater vent, and it's been O.K. ever since. Well, barely O.K. If we open our front windows, somehow it will cause a pressure drop inside the coach, and the fumes will come back at us from that cheater vent.
I don't understand why any manufacturer would choose to use an indoor vent for the plumbing system.
2016 Creekside 23RKS
2012 Ram 2500 Laramie 4X4 Cummins 6.7L