Bob wrote:
I just bought an old farmhouse (really good deal) an am going through all the
repairs that need to be done and now have a puzzle.

When a large amount of water is flushed down the pipes (i.e. bathtub emptying)
the basement fills with sewer gas. Found a clean-out cover rusted out (and now
fixed) but still having the gas odor.

I think that when the waste water enters the tank, the gas gets pushed back.
After the fix it has dropped noticeably, but it appears it has now been directed
to the tank vent.

There's still some gas in the basement, and a fairly good stench from the septic
vent, have I been living in the city so long to forget that this is normal?

Not normal. If you haven't had the septic tank pumped out, it may be backing up.
You should have it pumped out AND have it inspected to make sure it is in good
working order. It's possible you may need a new septic tank AND a new drain field.