I have a "08 30 BH so expect things are the same. Yes, you have to "open the box" which is just a frame around the heater by loosening the screws attaching it and sliding it to the left. You'll immediately see the two valves. If they're in the vertical position they're closedfor winterizing. Horizontal is open for filling & operating the heater. As far as adding antifreeze, remove the small panel under the bathroom sink to get to the pump. My trailer came with the winterizing option meaning the pump has a valve and short hose which goes into the antifreeze bottle. If you don't have this setup get it at Camping World. Make sure you have the water heater valves in the vertical position as you don't want to fill it with 6 or 10 gallons of antifreeze. Follow the instructions in your Sunset Trail manual. You'll find it's a quick and easy process and takes approximately 1-2 gallons of antifreeze. Ironically, I had to rewinterzie last week coming home to snow and freezing temps in Colorado.
Happy camping, Joel. You'll love your ST30BH.