According to :
True, I was advised by many people to be careful about the fumes and
definately to have someone watch over me while I was inside. Though
when the truck pumps out the liquids they are replaced with clean air
sucked down through the hatch. Just to be safe I opened the other
(outflow) hatch and ran a fan for 15 minutes down the main hatch.

Glad you were aware of the hazard. Some people aren't.

Doing a google search for "manure pit asphyxiation" yields altogether
too many examples.

We had one near here a few years back like the one described he

http://agsafety.tamu.edu/MANURE%20PIT%20SAFETY.pdf

[5 deaths]

While this is a lot less likely with a residential septic tank,
it still needs to be treated with considerable respect.

Methane is the most obvious hazard, but you need a fair amount of
it to displace enough air to asphyxiate or become explosive.

H2S (hydrogen sulfide) is a different beastie. It's very
toxic even at quite low concentrations. While the rotten egg
smell is an obvious indicator of its presence, a little known
fact is that concentrations of H2S high enough to kill you have
paralyzed your sense of smell. So if you smell rotten eggs
getting stronger, and then the smell abruptly fades away,
_don't_ assume that the H2S is gone - drop everything and run!

[In other words, as long as you can smell it, you're probably
okay.]

Then of course there's CO2 and Ammonia potentially present...
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.