i had an almost identical situation this weekend - first time out since last winter.
my water stopped leaking out of the underbelly when i turned off the city supply - but the pump would prime to draw water into the trailer when i went to tank supply. the pump wont stop running, it just bubbles water and constantly runs - but doesn't pick up from the fresh tank or dump in the grey tank.
I'm thinking a cracked waterline somewhere in the behind my p1 nautilus water system - wont allow the pump to build pressure, but leaks into the underbelly. mine goes in to get fixed int eh morning - warranty coverage. ill let you know they find.
can i piost a video here? if so i have a couple of my issue.
2018 DURANGO 2500 343MBQ | 2019 Silverado 3500 DRW Duramax