Mechanic couldn't find gas leak, now what do I do?

You don't mention WHERE you found the puddle of gas, was it under the tank? Along a fuel line? Or under the engine? The leak could just be from one or more injectors, and only leaking on occasion.

It's hard to say, we park in our driveway, sometimes back in, other times just pull in. There was snow in the driveway, and that's when my husband noticed it, when he was shoveling snow. The snow was wet and slushy, so it was difficult to tell exactly where it was coming from, and because we dont always park in the exact same spot, that makes it even more difficult to tell.

A leak in a fuel line, fuel filter, or pump area would be pretty easy to find, so I would expect any mechanic worth their salt would find it without too much trouble. A leaky injector shouldn't be too difficult to locate as well, and you may also see misfiring if that is the trouble spot. The EVAP system could be a hard place to find a leak. My suggestion is to try another mechanic. Of course some problems are hard to pin down, until they get worse.

The engine light came on quite a while ago for an evap leak. The mechanic blew smoke through the system and only found a few pinholes in the seal for the gas cap. The part was replaced, but the light still kept coming on for evap leak. Mechanic said older trucks like mine tend to have the light keep coming on, even after the problem is fixed. Perhaps there's something he missed...I'll take it to another mechanic. Thanks for the input!

Maybe the gasket on top of the tank is leaking? Only way to tell is to drop the tank. Like you said, have another mechanic look it over.