1995 Chevrolet Other truck issues

I have a 1995 Chevy truck 1500 5 speed. Every time I start it, it sounds like its about to die out and after leaving it running and revving the engine its ok. But when I go on the highway and I put it in 5th gear it just slowly dies out and then I have to put it in 4th. And not only that its blowing out white smoke from the exhaust and also smells like gas really bad, which I think might be coming from the engine. There's no leaks, and I already took it to the shop and the mechanic there did the distributor cap, checked everything he possibly could, map sensor, distributor, temp. Sending unit, spark plugs etc, and he couldn't find the problem, could that mean that my engine is going out?