Had exactly that problem with my Vegas 24.1. Debris was caught in the check valve in the water pump. This check valve allows water to flow one-way through the pump (from the fresh water tank to the cold water line) when working properly.
The debris allowed the check valve to be open partway, and when I turned on the city water, instead of meeting a closed check valve in the water pump, the valve allowed water to flow from the cold water line in reverse through the check valve, the water pump, and into the fresh water tank.
Like Jamie said -- I turned off city water, turned on the pump and opened the cold water valves on my bathroom vanity and sink. Ran the water for about a minute or so using the pump. Closed the vanity and sink faucets. Turned off water pump. Turned city water back on and no water was back flowing into the fresh water tank.
Ed & Bev Felker
Retired USAF Col and retired Nurse
Traveling with Lily & Bella ('Teddy Bear' breed)
2015 Vegas 24.1 (E-350)