Me still thinks you need to check for water damage in ceiling, walls, floor, behind cabinets.
Mold and mildew can put off an odor that can come and go according to humidity and moist conditions. I had a leak in the metal siding on the front of my previous trailer. Started leaking around the window seal. Got the insulation wet, then the wood, then mold and mildew, inside the wall.
You would never know it unless you took the window out and looked down the inside of the wall. Hope this isn't your problem. It can be a me$$!