Anything can be done if you throw enough money at it. It is a matter of cutting in another roof opening (fairly simple). However, there needs to be structural support for the AC unit (difficult). Also, you need to run power to the unit (difficult).
Have you checked if the existing AC units are working properly? Are they charged correctly (overcharged is as bad as undercharged)? Get it checked by a good HVAC tech, not an RV dealer unless they are someone you trust to do good work. I have seen many AC systems checked by an RV tech and they just charged with refrigerant rather then looking for the true cause of the loss of refrigerant.
If the units check out properly, then you may want to look into going to higher capacity units to replace your existing units.