Crank no start issue?

Car was running and randomly acted like it was going to shut off but went back to normal and next day it started and did the same again, left the house and it shut off at the light and didn't start after that had to tow it home. It cranks no start, replaced some relays, starter, spark plugs (they smell like gas I'm guessing from all the cranking and no spark?) Crank sensor replaced. When trying to start it cranks but the battery drains right away after a few attempts, and it cranks slower so that it dims all the lights including the car I'm using to jump it. Checked wiring and it seems fine, nothing was done to car so it can't be wiring issue? I'm trying to check ECU but don't want to buy one (pricey ofc) and for it to get damaged or still no start.