Does this happen only when you are connected to shore water?
If so it happened to me on a two year old coach. My problem turned out to be the check valve in my two year old Shurflo pump.
I emailed Shurflo to ask a question and Shurflo volunteered to send me a replacement check valve. I installed the new check valve and my problem was solved.
Check valve kit's differ depending on the make and model of your pump. You can internet search for a check valve using the make and model number of your pump. The search should bring up pictures of the check valve that fits your pump.
The check valve is easy to install.
Safe travels. JD