I am going to share my $0.02, Theory or Opinion with the Forum.
Gary may come in with a mathematical formula and 4 real world stories and blow this Theory of mine right out of the water into the Fake News Category of reply, Who Cares, Here goes?.
I have a 32 ft RV, 2 sinks, toilet, Shower, Outside Faucet.
Being a new RVr and from a temperate climate I had blown the air out of the lines carrying water to the faucets, toilet a couple times due to a slight Quick overnight freeze right at 32F for a brief period of time, didn?t want the freeze to expand those plastic elbows and Coombs clamps on those plastic water tubes cracking something with the expansion of freezing water and plastic joints that will pop.
Now the next follow on trips where I air voided the lines (Only) I was able to re-pick Prime on the pump just by refilling to 100% capacity the fresh tank. Having a full Fresh tank and the Hydro-static Pressure Weight from the Height of the full tank plus the 8.3 LB per gal weight of the water to press / void the air out of the pump and lines from being ?just slightly? empty there was enough in the system to reprime easily with this tiny pump.
Doing this slight purge of the system pre freeze the Pump would immediately pick up next use and shut off not continually running as I always leave with a full tank of water just for good luck.
After a few Blissful Adventures, a DEEP SUSTAINED RARE FREEZE was forecast by Mr. Weatherman.
I had become a little smarter and I learned I need to pull the the nylon plug on the Atwood h20 heater in addition to void out the system as much as possible with air also to avoid ice expansion within the day tank on demand, but this dropped at least another 4 gallons of water totally voiding the vast majority of water for sure and blowing 50 LB PSI pretty empty the lines of the system. With minimal water retention, if any remaining.
I did note that location of the H20 Heater plug seemed to be rather high mount location when pulling the nylon plug, "Up Hill" so to speak......due to gravity and the path water would naturally take if left to flow at will from source tank. Tank seems higher that the height of the fresh water tanks ability to push and totally press out the air to full prime the entire system or pretty even at best.
Remedy I feel Works: The two times I pulled the Atwood Water Heater Nylon Plug, Not sure what size 4 or 6 gal without researching but with that additional void of water creating a Air pocket to purge in the system possibly a slight air leak on the diaphragm on the suction side of pump to air out and Prime the motor would NOT cease running trying to achieve prime. Pumps don't like air in the system.
The thing that worked for me twice to stop the Constant Run of the Pump was to fill the tank, attempt to air out the system as before. Once seeing the motor was continual run mode.
Reconnected the city fill City Fill and Turn on RV Elec water pump while the water valve is seeking the city source, I feel flooding / pressuring the system, with the PSI from the city pushing / assisting the Running Pump airing out the system by playing with the valves letting the areas clear the air out the lines at each disto point.
It worked to keep the prime and motor operating and shutting off as it should twice when I was eXperiencing this. Luck. If so I am batting a 1000% 2/2.
The motor is only running to supply water, regain the prime and shut off. But with the partially primed system and lots of air to press out for this Tiny, Plastic Pump I don?t see it, self correct when dropping all H20 from System. Too much volume too small pump.
I would Try forcing City source H20 In Concert with a Running H20 House Pump Elec Motor Engaged. Go to all sources Airing the valves Outside (H/C), Shower (H/C), etc?..get them all toilet like with City tag teaming the pumps action.
Then once the flow seems not to sputter........take off the city pressure augmenting the pump and see if it will quiet down. I think the system needs to purge the air.
Also adhering to the KISS principle of life: Did it correctly work before? What happened since then? What did I possibly do to screw this up?
John