ACC cable tap for DC-DC chargers, okay to use the trailer connection?

[overl4nd]

I am planning to install a 40A DC-DC charger. For the ACC wire, instead of tapping some fuse box inside the vehicle, is it okay to connect it to the 12V positive on my trailer wiring harness? It comes on only when the car is on.
I was also thinking of not using the ACC wire if i go with a 20A charger as I can pull 30A from the trailer connection. Thoughts?