I also have a 2013 F21. I just replaced my drain master valve last Thursday for the lake last weekend. I ended up using a manual valve because drain masters customer service is sub par. I ordered the gate on a Friday, website claims same day shipping if ordered before noon. Anyways I never got the valve (waited a week) so I have a manual one still installed. I will be installing the new drain master in place of the Valletta manual one I installed. Im kind of rethinking my decision and maybe should have replaced both with new vellettas. Either way I'll be keeping the manual valve in the boat as back up.
My only word of advice is that when replacing the valve take the 4 bolts out of the valve which will allow you to remove the flanges and slide the valve out. You will have to insert the gaskets back in the flange ends but that seemed easiest to me. I thought I was going to have to remove the trim ring on the outside of the boat to allow room to remove the valve with the flanges but then found out that removing the bolts was the way to go.
Good luck!