My thoughts won't be much help, but they may trigger additional thoughts from you. When you flush, you are pressurizing the black tank since the toilet inlet pipe is much bigger than the vent pipe. When not flushing the tank is at a static pressure and any vapors can take their sweet time to exit to the roof. If the assumption is made that there is no crack on top of the tank OR the vent pipe entrance into the tank is solid and can only vent through the pipe...could you be getting sewer exhaust being layered close to the ground and being pulled into the basement where you smell it? Maybe the also slim chance that vapors are being forced out of the black tank throught the black tank rinse line and entering the basement area. Good luck and hope whatever you find is easy to solve. Jphn
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2012 F350 6.7 L dually, 2013 3800RE with 6 pt leveling, Sumitomo 17.5" load range h tires, Samsung 18 cu ft residential fridge, 8k Morryde I.S. with disc brakes. Full timing since 2012.
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