re Power vent...for a closed tank area, even with an outside vented cap..
.standard procedure used to be.....one opened the hatch to the tank; turned on the power vent ( which was a special deal since its wiring was, somehow, not exposed to the fumes.)
Then, run the vent fan for FIVE MINUTES....followed by sniffing the area for any gasoline fumes. If any found, run the fan again (and again) until perfectly clear.
only then...Start the engine.
That was SOP on a 27 ft. Catalina sailboat with an Atomic Four gasoline engine 20 yrs ago.
R