A/C drawing air up sink drain.

Remove the grill from the A/C and look up into it. Is the attic sealed off from the return??? You should not see any insulation or framing for the roof. If it is not sealed off completely, you can draw sewer gases through the attic from the sewers vents in the roof. The vent pipes are not sealed from the roof and just stick up just enough to be covered by the vent assembly.

Than RV AC should not create vacuum. I would check for a cut in blower housing.

I would check the sink's air vent. This device is designed to allow air into the drain line (grey tanks smell just as bad as black tanks). If the vent flap is bad, it will allow air to exit the drain line into the unit. The vent just unscrews to remove it.

I once had a problem similar; it turned out the tank vent line through the roof wasn't well sealed, and stinky air was entering through the cracks. Stuffing fiberglass in the cracks solved that problem.

Doug

(a) perhaps the inside vent under the sink doesn't seal properly. They contain moving parts and although they are supposed to last forever, they don't.

(b) you may need to adjust the air flow through the AC. Perhaps it needs some air door opened or closed a little more?

(c) maybe you could leave one of the windows over the sink cracked a little bit to relieve the pressure.

When you fill the sinks with water, and run the AC there's no smell?

AC does not create a pressure difference inside the RV, it doesn't vent air in or out, it just circulates it through a coil.

The air vent under the sink lets air into the piping not out, although it could be faulty and not doing its job.

My guess is the source of the smell is someplace else, and the air movement from the AC is stirring it up... or maybe there's something wrong with the AC itself, refrigerant leak maybe?