2003 Dodge Caravan Van won't run now!

Engine Mechanical problem
2003 Dodge Caravan 6 cyl Automatic 116, 00 miles

My van began hesitating at during normal highway driving and someone recommended I change the fuel filter, did that but it didn't correct the problem, the check engine lite code said the map sensor was bad, so I had that replace, then it started smoking (gray) after starting it and sputtering, the spark plugs were "black" and the van was running very rich and that I would need a cat converter, replaced all of that and it still isn't running, the last mechanic said the computer needed to be flashed, took to dealer for diagnostic and it wont take a flash or find anything in the computer - now trying to say they need extended diagnostic time to look, Any Ideas? Please help - this is a van conversion with a dropped floor for my son's wheelchair and we desperately need it and have already spent a significant amount of money and still no resolution?