The problem with being gone for so long is that if something dies and you arent there to remove it, it will rot and could cause your system to crash.
You could have a coral only tank and be just fine as long as you keep your lights on a timer. Its possible you could get away with some fish too, although eating once every four days would piss them off.
They make automatic fish feeders, but if your more interested in coral, then fish would make for a really unnecesary challenge.
I would be more worried about my tank spilling or something and not being around to do anything. Another issue would be water evaporation. You can bet that will present a challenge trying to top off the water. If you dont, and too much water evaporates, then your salinity will rise and could anger your tank.