Try half a cup of baking soda and a half a cup of vinegar down your drain, leave sit over night. Then in the morning flush the drain with hot water for about 30 seconds. The flies are actually there because of the residue on you sink pipes. If you clean them up, which the vinegar solution should fix, the flies should go away.

I don't know what the problem is, as I had them in my truck last week, and I don't have city water in my truck.

maybe don't use your sink drain as a garbage can. Washing food scraps down the drain and then not throughly washing them down the drain will bring out the fruit flies.

Just under your sink there is a pipe shaped like S or a P. It's called a Trap. It's designed specifically so that it always has some water in it to block the pipe. Check out this picture: http://bit.ly/Q0Haoe

The idea is that it's preventing sewer gas from coming back up through the pipes and into your home. In the same way, there's no way flies are coming up from sewers because there is a section of water preventing them from doing so.

If there are any problems it is on the small section of pipe between the sink and trap. There is likely food, most likely fruit juice reside on the walls of the pipe. Use a chemical cleaner or take off the drain screen and physically clean this pipe and the flies will have nothing else to eat. They will hang around for a few days until they die off.