Hello, I would recommend a small real marine AC, there is a company called webasto, They make a 4500-5000 BTU unit that works off 12V, it is a 110 V unit that has a built in inverter. It will run around 5 hours fully on, in 2 group 27 batteries. They work great not only will cool the cabin but should be able to sleep fine. I had a 2600 martinique with a over the latch unit you can get them for 500-600$ all day long. On top of that you will need a 2000 KW quite generator, $1000 so. For that much you can put the real deal and use it underway. The over the hatch unit works great but is somewhat hard to carry, it takes a lot of space on the boat especially on a 24', and not really a secure way to fit it. I hope this help!
http://staging.itrmarine.com/AirConditioners/FeelCoolFastDV.aspx, you can buy the unit WO the inverter and get a cheaper inverter. They supply a REALLY nice XANTRE unit, so i would recommend buying the unit by itself and get yourself a nice inverter. The unit alone and pump should run you arround 1200$ or so!