Here is a pic of what mine looks like after the connection port. Sorry it tilts to the left. The bottom of the pic is the right side. Mine has an elbow where the backflow preventer would normally be but the previous owner removed it so I use an external one.
As you can see, it's installed so that no water is retained. Water drains away from the backflow preventer on both sides - back out the inlet port and down to the flusher head. It wouldn't hurt anything to use low pressure air (under 30 psi) to blow it out.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)