As Adrian says, it's the moisture that causes the main problem in a caravan cupboard.
Make sure your selected cupboard is strong enough to support the substantial weight of a microwave, particularly during transit. If you're fitting in a high level cupboard, this will be a lot of weight high up, which goes against the Towing Code.
Line the inside of the cupboard with stick-on vinyl "Fablon" to prevent moisture being absorbed into the structure of the caravan.
Cut vent holes and fit a small extractor fan to the selected cupboard (and ALWAYS use the fan when the microwave is on). This is vital if you reduce clearances below 50mm all round on domestic microwaves. This step seems to be omitted by caravan brands which fit microwaves as standard - and then they wonder why they get so many complaints!