[JRTJH]

The OP might have resolved his problem, hopefully he has. It would be appropriate (and kind) if he did resolve it, to post the fix. If he hasn't, then by "logical deduction" here's how it goes (the way I see it and by most who have responded)...

The specs for the trailer indicate 30 gal black and 60 gal gray. Traditionally, Keystone installs two 30 gallon tanks, one for the bath and one for the galley when they list the specs that way. I have not seen a Keystone trailer with any single tank (other than fresh water) that is larger than 48 gallons.

Now, the LOGICAL CONCLUSION to the problem and solution are:

There are no problems with the bathroom (OP stated he dumped the black and gray tank) and those drains work. The galley sink won't drain. So, from all appearances, as most have responded, the galley gray 30 gallon tank is full. We have had this happen to a number of new owners in the past and it's well documented on the forum.

The fix: FIND THE GRAY GALLEY TANK DRAIN. It's under the trailer, either mounted in the frame rail or in a small slit in the coroplast. In either case, there is a 3rd dump valve handle that the OP has not yet found (or if he has and not told us he found it, hopefully he will update the thread).