We did this in our Uhaul
fiberglass camper, took icebox out, put a dorm fridge
in. I first made a platform for the fridge to set on. That raised it a few inches off the floor. I fastened the platform to the floor with common hardware angle brackets. I used more of those brackets in various configurations, flat, angle, straight, to secure the fridge to the platform. I just figured out what shape bracket would work on each corner of the fridge and secured the bracket with machine screws or bolts. Then used screws to attach the brackets to the platform. Each brand fridge is different so it's see what will work on yours. A mini fridge generally has a compressor so read the manual and leave space for air movement around it.