The Techs have told me that the way the flexible pipe is routed, it is creating another "P-Trap" behind the cabinets. I intend on eliminating this (If I can). I think that portion of the drain would be very difficult to winterize effectively.
We didn’t have a problem with our sink draining on our FE but it wasn’t fully secure and ended up on the heater and melted. Of course this caused it to leak everywhere. It may be your drain pipe is getting binding up or has collapsed some. The drawer under the stove allows you access to the pipe. Good luck the pipe also goes through the side of the slide wall and into the bathroom so there are a couple of places it could get pinched.
I have a 2019 Four Winds 31E. Noticed that my sink was draining super slow. Did some detective work and found that the flexible pipe does indeed cause a trap of its own. I have secured the hard line with zip ties the best I can to improve the draining. The actual issue is where the drain goes into the grey tank. The pipe that comes up vertically out of the tank is too long. I plan on shortening it by about three inches and that should solve the issue. Iím fortunate in the fact that this is all underneath my bottom bunk in the slide out and is easy to access.