The Lighthouse has availability. This is actually closer than the Sandcastle Suites in terms of walking to the park. From SS you have to walk through Camper Village which is where The Lighthouses are.
It's a bit more than $100/day, but you get free tickets into the park and Soak City (the waterpark). Parking isn't cheap either, so that's something to factor into the price as well.
EDIT: They all have availability.
