One of the important things to list with questions is what exact RV you have as things do change from year to year!
Can we assume this is a 2021 model?
Based on that guess, I went to the prats catalog which has a super great system to look at the interior of our RV and see where things like lines and tank set.
https://catalog3d.winnebagoind.com/menu/Parts.htm
I t shows that on the 2021 model the vanity sink drains into the black tank and the rest all goes to grey.
I did snip of the lines into and out of the fresh tank and it looks like the question of kinks, etc should be a small one as it is so short and the way it ties into the lines with the pump, making when the pump can feed good water pressure and flow, the drain should be very close, too.
I've marked the vent as coming from the bottom but if the pump is able to provide water, the blocked drain can't be the cause of not draining well as the air has to come in the same spot.
When water comes out to the pump as marked in blue, it meets a tee to go to the drain, so the odds of kinks there seems remote?
However, it might be easy to see a plastic drain valve turned the wrong way may not be opening now????
But keep in mind that this is guesswork and based on the 2021 model, so if you have different, things may be different!