A common problem that can cause this symptom is the pump sucking air on the intake side. Check the connections on the intake hose for the pump between the pump and the tank as well as the line itself. Since the trailer has experience frozen pipes, check the pump carefully as well, it could have a crack that allows air in.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill