I have never heard of anyone doing this. It might work if you leave the lid slightly ajar, or prop it up slightly with a wooden spoon. Otherwise, there would be no way for the water to evaporate. I don't think you could do many at one time, either.

Seems to me like the tomatoes would steam more than dry - things tend to get wetter in the slow cooker. I've never done it, but I'd think that if you wanted to save gas, you could preheat the oven high, and then turn it off and then leave the tomatoes in there a long time, as if they were drying in the sun.

Slow cookers are designed to keep moisture in, so I don't think this approach is likely to work. On the oven's rack works because moisture leaves in all directions - whereas a crock pot would only allow air to escape the top (assuming you left the top off). And I'm not sure you'd get much energy savings if the crock pot's lid were off.

I agree with allmentable - Repeatedly heating the oven, then allowing it to cool, might be your best bet.

I think that this might not only be a bad idea, but it might be dangerous. I once had a roast dry out in the crock pot due to a lid that was accidentally ajar. The ceramic insert in the crock pot overheated and cracked in half, ruining the device.

I am a big fan of drying tomatoes, and used the oven method for years until I found an American Harvester dehydrator at a consignment store this summer. It works like a charm and I've been drying everything since I got it. I know it's a unitasker, and is bulky to store, but they are easy to find used. There's nothing like using home-dried tomatoes in the dead of winter.

No, because there the moisture is retained using a slow cooker.

what about a toaster oven? Don't forget that the electricity used to power the slow cooker was generated from a raw source - most likely natural gas. The generation of electricity from raw sources is about 30% efficient - meaning you're wasting 70% of the energy! And then there's efficiency of transmitting the energy from the generation facility to your slow cooker (another 90% reduction), and the efficiency of the appliance itself (another ~50% reduction).
Ovens are usually considered "inefficient" because of the large internal volume that's filled with heat. Unless you have a small batch of tomatoes you want to dry, the oven will likely be way more energy efficient than any appliance. (meaning cheaper usually, too!)

A toaster oven is a good idea or if you really love dried tomatoes and other dried fruits and veggies pick up an inexpensive food dehydrator (about $40 give or take a bit). It's warm dry air that does the work with drying tomatoes so like people have said above a slow cooker is made to hold in moisture.