Hi from frozen Minnesota. I often drain my fresh water tank with the pump. I have that "winterizing port" on the inlet side of the pump where I can draw in RV antifreeze to the pump and suck down a gallon or so to fill the fresh water system with RV antifreeze and thus protect it from -3 below zero like this morning. When I open the port, I "assume" the little bit of water in the inlet tube from the fresh water tank to the pump drains back in to the tank.
I am not concerned about an inch of water in the bottom of my fresh water tank freezing up over the winter. So I don't open the cheap plastic drain valve that is stuck anyway.
Your photo shows an old plastic tank with "spin welded" fittings, a very common practice with molded tanks. That plug in the 2" port may be good and stuck. The spin welds won't take a lot of torque. So if you can break it free without much force, go ahead and plumb in a petcock drain. If not, then leave it. It is not worth cracking your tank to install a drain valve that you really don't need.
By the way, there is no charge for my advice!