2000 Chevy Astro gas smell and msg light

I’ve had an interesting last few days. My van runs but eats gas.8 MPG we still don’t know (we smell gas) if it’s a leak or even where the gas is coming from. New to me on the 8th On the way home it work fine, the next day I turned it on and seen the check engine light on. They took of bat to clean it.I had to go to work, I had 2 contracts from dish. I got the 1st one done and the van died before I got to the 2nd one.I had to sit in the van 27 ° out for 3 hrs before someone could buy $115 in battery and vise grips. The other one was dead and it worked but light was still on. The 9th I checked what was the error code and it said that it was the 5th cylinder misfire so we (myself and brother) put new spark plugs in, lines and cap for $143, the worse part is we found a paper that said that they had work done in oct 08 for the same thing that I just did. So it has to be something else that cases the same problem. If its not that then. What? We put it back together. The van will run it just when you stop you must push on the gas and brake to keep it running. It dies at stop lights and stop signs and with 8mpg, has white smoke out the tail pipe that has people waving their hands fanning it away at a gas station. I was thinking it now above my level of maintenance I have to get a diagnostic to see what’s going on.