For winter I dump both tanks as usual. But the manufacturer put a second black tank valve (winter) very close to the black tank. That is the one I close during winterization when I am dumping. On my rig the three inch pipe runs all the way across the camper from the tank to the “summer” valve. I suppose there are two reasons for the two valves. One is if something hits the pipe and breaks it there is a valve close to the tank you can close. The other is while winterizing the shorter the pipe the better to get pink stuff into it.
I have always put a bunch of pink stuff into all of my drains. About half of that does go into the black tank.
On all of my RVs I have never known were the water traps were. So I errored on the side of caution. Never had a problem up here in Wisconsin. Eh, it do get cold up here.