looks like it IS the galley tank after all......found the air admittance valve loose, not even finger tight...replaced it. Checked the galley vent pipe, 2 paper wasp nests although not totally blocking it...maybe 50%. The stench I was getting was evident in the outside kitchen sink drain (has a mechanical trap rather than a p-trap) also which sent me to the galley tank. I gave the galley vent a good blast with the garden hose.
I was always in the habit of completely emptying the galley tank when breaking camp...guess I need to have some water/chemicals in all 3 tanks to keep the gasses from building up. Fingers crossed!
2005 GMC Sierra 3500 SLT DRW 6.6 D/A
2013 Crusader 335BHS TE