I have a Grey Wolf not a Salem, so couple thoughts. Assuming you don't have a winterize connection on the outside of the TT, then look for a bypass valve on the intake line going into the water pump. Probably close to the screen. If your TT doesn't have one, you'll need to install one.
Mitch in MN
2018 Patriot (Grey Wolf) 23MK travel trailer
2011 Durango 5.7L and OEM tow package