With a lot of air in the system , the amount of line pressure required to have the pump shut off might not be reached .
Is this a new issue or an issue on a new to you RV?
Can you see the pump, checked for water leaks ? Has it got a clear strainer on the inlet side ?
If not properly winterized these strainer housing crack and allow the pump to draw air . There are a couple of types , picture below is one , these aren't supplied with a new pump so if you want to replace the strainer and later find you need a pump you haven't spent too much extra.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.