I don't know if it has a formal name, but I've seen it referred to as a "riser", for obvious reasons. The join between the riser and the roof surface must be well caulked.
As for the join between the top of the riser and the air conditioner, I'm not sure that this needs to be caulked. After all, rain is expected to get into the air conditioner housing through all the openings and slots in the air conditioner case, so there must be a pan in there that catches that water (along with the condensate) and directs it out to the surface of the roof, where it runs off the side of the trailer.
Bill
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