Four Season Truck Campers with a Slide- really four season?

I camp a lot on winter with my motorhomes that had holding tanks just hanging under the floor. We don't have extreme freezing in CA and daily sun warms up everything pretty nice, so I never had freezing issue.
When I was trying to keep grey water tank low, my observation is that the black water will not freeze even with moderate freezing.
You can always dump some antifreeze there.