No, in my opinion it would not be safe to replace your built-in Microwave Oven with a Toaster Oven. To the best of my knowledge there are no Toaster Ovens that are designed to be built in to cabinets.
Even the best insulated Toaster Ovens need a lot of breathing room to dissipate the heat that that they generate.
If you try to put one of these Toaster Ovens in the space that was once occupied by your microwave; you risk starting a fire in your RV. Even if you do not start a fire in your RV, you could end up with heat damaged cabinetry.
My suggestion to you is to keep the Microwave and get a Toaster Oven that will fit well on your RV's kitchen counter.
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