If your black or grey roof vent pipes coming from the holding tanks aren't tall enough to protrude through the roof into the vent covers you might be getting some smell in between the roof and ceiling.
I had a stink occasionally and couldn't figure it out like you till I finally pulled the vents covers off, grey pipe was 1" shorter than the roof line. I added an adapter on problem solved.
12v 500ah, 20 cells_ 4s5p (GBS LFMP battery system). 8 CTI 160 watt panels (1,280 watts)2s4p,Panels mounted flat. Magnum PT100 SCC, Magnum 3012 hybrid inverter, ME-ARC 50. Installed 4/2016 been on 24/7/365, daily 35-45% DOD 2,500+ partial cycles.
