Get a pizza brick or a 12 inch unglazed tile. It acts like a hear sink and works wonders keeping the temps even. That and careful placement of food should take care if your problem.
Yes, use pizza stone or what we use is a stoneware jellyroll pan from Pampered chef that fits the inside of a toaser oven. We don't use the pan at home so it was just in the camper and we repurposed it.
I usually lower the temperature and/or shorten the cooking time. Seems like the oven is small enough that it may condense the heat so things seem to bake/cook faster. I've had to do that with most RV ovens I've used.