We have a 15' Skyline Nomad travel trailer with the AC in the wall, not on the roof, and the cabinet right next to it gets wet when it rains. There is a large vent on the roof and a grill on the side...
I would pull the AC unit out and try to find out where the source is. Then go outside and find cracks in the seal, missing seal, anything that might let water in. Water can come from almost anywhere as long as it's going downhill. If you're not doing at least an annual check for leaks you're likely to be missing something.