back filling water tank

[wingnut60]

Yesterday, wife heard water running while taps were off. Discovered that the fresh tank had filled and was overflowing. Have determined that the water supply is back-flowing thru the water pump. Understand there is a check valve to prevent this and probably has failed. Need to know where it is located. Only item not of plastic hose and banded fittings is a brass tee on the pressure side of the pump maybe 1 foot downstream of the pump. One leg of the tee is feed from the pump; one leg is feed from the filter/muny water supply; 3rd leg goes into the pex supply line that I assume feeds water to rest of trailer. Is this the check valve, or is it built into the pump?
Any help will be much appreciated. Many miles from RV shop of any kind, but can work off of water tank and keep city pressure off the system.
Thanks,
Joe
Pump is the 5.7 Smart Sensor, has worked well for the last 3 years. And plumbing is the original rats nest, not the manifold now in use.