Should I use my toaster oven for slow roasting?
In the question body, you say that you aren't trying to get roasted tomatoes, you are trying to get dried tomatoes. This is a very different process from roasting. Toaster ovens are great for roasting stuff, but regular ovens are much better at drying. 

To get a nice texture in your dehydrated vegetables, you want to be as gentle as possible. You are trying to use very dry air, and just enough ambient heat to evaporate the wetness in reasonable time. Blasting the tomatoes with heat or scorching them is exactly the opposite of what you want to do. 

But you see, toaster ovens are very much on the "blasting" side. They place the food close to the heating elements, which are frequently exposed, and let them heat the food pretty directly. There is much more direct radiation in their heating than there is conduction from the warmed up air. In contrast, the regular oven achieves an enclosed space in which the air is hot itself, and it conducts its heat to the food. Also, the oven walls warm up somewhat, and the top and bottom are much more evenly heated. You have gentler heat coming in from all directions there. It gives you a much better drying environment, at the cost of worse roasting results (less crisp/browned surface). 

So, if you want to imitate a dehydrator (for which an oven is already an imperfect substitute) at least do it with a regular oven.