I occasionally have the dry bowl issue with my Thetford 301. No big deal. I don a pair of latex gloves, coat the upper and lower sides of the seal with vaseline. When doing this I move the seal edges up and down with my fingers to make it pliable. This seems to keep the gasket soft and working well for a few months, then I repeat this process as needed. I have not yet found a need to replace it.
As far as flies, I can't say I've experienced that. You may want to do a search on this forum on black tank flushing and give it a good cleaning. I use nothing but water (and lots of it) to maintain my tank. No chemicals are necessary for me.
Steve
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