Some air conditioners will run on 15 and 20 am circuits, I have one rated at 12650 btu and it will run fine on a 20 amp. According to the specs it should also run on a 15, but it was getting up there where the starting amps could be iffy. Check your manual or the AC literature, there should be a table that shows the power requirements for starting and for running.
Anything approaching 1500
will need a 30 amp. A 13500 with a hard start capacitor should be able to go on a 20, but check the specs to be certain. Most 13500 should be on a 30 amp circuit.
You could also have an electrician put in a 30 amp service plug and just use an RV converter plug. It is not a difficult task for them and they can put in a weather resistant lock box for the 30 amp service.