I have the same problem. Could be the Anderson valve or the check valve in the water pump.
Mine was also intermittent. So, Anderson sent me a new valve and it "seemed" to stop. Then it reappeared again. This time I am changing the check valve in the Shurflo water pump. $10 so we shall see. I have not done it yet. If that doesn't fix it I will do what another said above in this thread and get a barbed in line check valve (I actually did once but it was for PEX and y lines for the water pump - where I was going to install it were not PEX so I returned it and ordered the water pump check valve. NOTHING is ever easy in an RV!)
I don't make it much on here anymore so will not see your reply to my comment. Drop me a direct message and when I come back on I will see it.