I have experienced potable water tank being filled from the city water side. I shut the city water off and used my pump until the level indicator was back to one light. Now, two days later, I have two lights.

Is there a check valve in the system to prevent back fill of the tank?

To the best of my knowledge there is not a factory installed check valve in the system other than what is in the water pump.

I have experienced the same problem with 2 different pumps. Shortly after they started to back fill the fresh water tank the pump completely failed.

Some people suggested that there maybe some debris in the pump clogging the check valve and it may work out by flushing a bunch of water through it, but sounds like you've already tried that, so don't know what else to suggest other than put an inline check value in before the pump or get ready to replace it altogether.

In our 2013 RL there is a quarter turn inline valve on the suction side of the pump meant to isolate the pump from the tank during winterizing. Closing this makes the pump draw from the winterizing line located in the convenience center. I closed this valve after we noticed the water tank filled while hooked up to city water. Never got back to replacing the pump, we generally travel from campsite to campsite, and use city water. Ken is right, there is a check valve in the pump assembly, and judging from the number of complaints over the years on here, they fail quite often.

In most cases the check valve is a replaceable part of the pump. There is a manual in the Technical Library under Manuals in this website for the most commonly used Shurflo pump with a description of the check valve and how to replace it.

Mine has filled the tank several times, but after using up the tank water we might go for a month or two without the tank filling. Still can't understand why. It started a year use

We had the same problem, we also have a valve right at the pump and closing it eliminates the issue. We seldom use onboard water so this works for us

I just had this problem as well and did some research into the situation. According to the Pentair pump manual, the pump has a bypass on it that is set to begin allowing backflow when the city water side reaches 40 psi and maxes out at 62 psi.

I found this out because the park I am staying at has city water at 80 psi and my neighbor came over yesterday and told me there was water leaking from under our rig.

I have an adjustable psi regulator, but had never set it up before this event. I checked the Redwood manual that came with the rig, and there is no mention of max allowable city water psi.

So, I dialed my adjustable psi regulator back to allow a max of 40 psi in to the rig and the problem went away.

My regulator is set at 55psi & no problems. Normal city pressure will vary between 55-65psi & for me 40 is too low. Also on my pump there's a valve at the suction side you can close to prevent filling holding tank, just have to remember it before using the pump.
FYI, pex piping is rated upwards of 100 psi I think, but the cheap plastic fittings they used aren't.