I would start by cleaning the head of all dead sea critters that are under the bowl lip, rebuild head pump ( or replace head ) make sure tank vent is clear ( enlarging it also wouldn't hurt ) and use KO after every pump out.
If that don't stop it, check lines for permiation and replace as needed, then clean, scrub and flush the bilge
1978 Tayana 37
Freedom comes when youíre ready to sail away. True freedom comes when you donít have to return
Cut off from the land that bore us, betrayed by the land we find, where the brightest have gone before us and the dullest remain behind, .......but stand to your glasses, steady,.......tis all we have left to prize, raise a cup to the dead already, hurrah for the next that dies