In the past we stayed for several months in the same RV park in our MH. We used Exodor Bio-Pak drop ins for the black and gray tanks. Sometimes there was a horrible eggy smell when we dumped the gray ...
We've been camping for over 50 years, the last 5 full time. During that time we've tried many different 'odor blockers', and that's all they are. Nothing works better than plain old water. Let your black tank get about 1/2 to 2/3 full before dumping and use water to flush it clean. The same with the grey only usually never more than half. Of course, scrape your dishes in the waste basket before washing them. Common sense says not to put food waste into your grey tank....Dennis