Picture would be helpful?
On 1 of my campers factory build-in microwave died and I put smaller unit into the hole. Secured with ratchet strap.
That turn to advantage as bigger oven pushes inverter voltage drop, what comes with electronic beep, what makes my dogs going amok.
My Fleetwood has lot of countertop space, but I added under the cabinet "space saver" toaster/oven to it.
Turn out I use the oven much more than micro-oven, even Black & Decker screw up last design of the ovens and they burn food easy.
So it all is learning process.