My car won't star but cranks over

I have a 01 nissan and i bought it 2 years ago and it ran
great till a week ago. Last year i had to replace the battery
amd starter. Well about 3 weeks ago or 4 my car would
randomly sputter not only could i feel it i can hear it
coming from the exhaust sounded like a bad stick shift.
Then it got worse went from randomly to consistently and
at a stop or park my Rpms would drop down to 500! It
acted like it wasn't getting gas even tho i have half a tank.
When i accelerate it went slow but it sounds like the air
fuel ratio is off sounds like to much air. Anyways it stopped
starting all together even a jump start doesn't help and if it
does it runs 5 secs then dies and hitting the throttle does
nothing. In the past week i changed the fuel pump and
strainer, all 6 spark plugs and coil packs, and now the
camshaft sensor and still nothing! Oh amd when it does
start is smells like a lite exhausty smell idl how to explain
it. And my dad said the spark plugs smelled like unburnt
gas and the camshaft sensor was cover in oil idk if thats
normal. I cant use a code reader i have cause my car wont
stay on long enough. Im stumped. Help!!