Used to have that problem, was cause by my dumping sequence. I leave the grey tank open and dump the black when full. I used to close the grey before dumping the black and would get a horrible smell in the coach. Started leaving the grey tank valve open when dumping black and the problem stopped. Was caused by gas suction when the black tank air relief pipe was under the surface of the blank tank water level. Shouldn't be able to do that but it does.
JimF, CWO, US Army Ret, 100% DAV
2011 Georgetown 378TS
2008 Dodge Caliber Toad