Low point pressurized drain petcocks

Had some freeze damage last year,but took my camper in for repair at the dealership. Everything tested and help pressure. So...I was in the process of winterizing my 2013 flying cloud and upon draining the low point ball valves. Both snapped off. It's clear that they froze and all it took was turning the gate valves open to finally shear off. My question is what is the other end of the nipple connected to. I got an easy out thinking I could just back out the brass nipple but, what ever it is connected to is turning. Is this something that anyone knows how to do. As it seemed like an easy fix but it goes through the water tank. Any help is greatly taken.

Thanks.

Marcus.