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.

Most of the toaster ovens I've seen have a timer that doesn't go much further than 30min / 1h depending on the model, so you'd have to reset the timer every time it ends. It's probably ok to use, but it will be annoying to be monitoring your roast so often.

There probably won't be any temperature issues, except that these ovens also usually leak quite some heat over time so you might have to wait to do this roast in winter :)

Toaster ovens tend to not have very good insulation ... so although there's less to heat up, they leak more heat into their surroundings over time.

For the length of time that you're dealing with, I suspect that there won't be any significant energy savings (if at all), and the time to pre-heat is negligible compared to the total cooking time, especially as you said you're dealing with low heat.

A regular oven can be left mostly un-attended for the time, so I'd go that route.

I also like the regular oven for this sort of task, as I know I can fit my pyrex pan, and that works really well for roasting tomatoes and cleans up easily. (halve them, then place them cut side up in the pan, so the juices evaporate).