I installed a RV refrig. roof termination kit for my conv. microwave. It is a two piece plastic set, a roof riser, and a cap.
Does a fridge vent have enough air flow so the exhaust on the microwave works properly? A fridge vent is normally for a slow convection venting of heat. This sounds like a good idea.
Fridge vents are usually installed parallel to the length of the RV like on your bus. Do you think installing one perpendicular to the length of the bus will cause problems with it blowing off?