Usually the foul odor that comes from the holding tank areas is from the affluents remaining in the hoses over time and the smell comes through the hose. A test for this is to wrap the hoses with Saran Wrap. If the odor goes away then that is your problem. When replacing the hoses use the best you can buy!!! If the hoses are older than 10 years that is probably the problem.
When I replaced the hoses on our Island Packet 38 I used black plastic sewer pipe everywhere I could and used marine white hose where the plastic could not be used.
The odor was totally gone and I expect the new higher quality lines to last much longer.
Ray