Bathroom sink not draining

[Audiff]

I have a 2014 Jayco 22 FB TT and the bathroom sink is real real real slow to drain. My wife has been in it since April and the problem just started occuring. The gray tank is not full and the shower and kitchen sink drain fine, it is just the bathroom sink that has the problem. I checked the P trap and there is nothing in there. I am assuming that it is the vent pipe that is the problem. Has anyone else had this problem and if so what are all of my options. I'm trying not to take the cap off of the vent pipe.

[Fr8Train]

Check the vacuum breaker under the sink. I've also heard of some weird rubber thing in the pipe that can cause issues, and plunging just makes it worse. But I thought that was used in place of the p trap. Try a snake?