I stayed at the Mills House in March of 2016 and used rewards points. My grandson was born in Charleston March of 2016 (expensive place to hang out waiting for him to be born). I want to say around November 2015 I was able to book Feb and Mar. Some were at 50% based on timing of when I booked. Even at that I let the rooms go - the points are ridiculously high. As I let things go, I never saw them come back.
I chalked it up to WYN teasing with one night at the hotel so they can market that it's available to us. I've never watched inventory there since.
Mills House is an awesome location, but as many have already said, use Wyndham rewards points, if you have them or look in advance for good cash deals.
We've stayed in Edisto a few times, but it's a LONG commute to Charleston. Blue Green has the Lodge Alley Inn. I've picked that up on the rental board here before from other tuggers. That's very nice, too, and I got some very affordable rooms a couple times. For just paying cash, I've found good deals at Andrew Pinckney Inn. Overall I advise book early. Charleston doesn't seem to have enough hotel rooms. There is always something going on. It's expensive to stay on the peninsula, parking is expensive (I did not have nor need a car when it was just me hanging around, I did a lot of walking). Things fill up and last minute deals are hard to find. Prices go up, not down.
And ditto the comments on Galveston - same non existence of inventory whenever I've checked. Feels like a waste of time of our time to put these resorts on the website.
