We had a similiar problem with an odour that I was sure was black tank related. Did all the septic stuff adding chemical, flushing etc. When it persisted I was perplexed. The Navigator detected an odour outside at the battery door. We opened it to find that my oldest battery had shorted and charred as a result of one of the cells drying out. The battery was dated 2005 and that was last year, 2015, so 10 yrs old and well maintained.
I was sure it was septic odour in the basement area, Just a thought!
Married To The Navigator
2005 Red GMC Duramax 4X4 HD Crew Cab
4" Magnaflow, Banks Air Intake
Firestone Ride Rite Air Springs