I had the same problem. I used the water hose in the toilet to flush the pipe and the tank. but it still blew back. so I put the water hose down the vent on the roof and that did not work so I waited until the pipe backed up not sure why it did it because the tank was empty. When the pipe backed up I ran the toilet flush valve until the water was just a few inches below the gasket in the toilet and then I used a plunger and this has cured the problem. YOU MUST MAKE SURE YOU HAVE THE VALVE OPEN IN THE WET BAY BEFORE YOU USE THE PLUNGER. Fill the bowl up a few times to make sure it did the job then flush the tank. hope this will help you.
Ed & Annie full timers
1998 International 4700LP 4 door ,DT530,300hp 6sp.with a 4:10 ,air ride https://www.redwoodowners.com/attachments/signaturepics/sigpic1866_1.gif[/IMG]