I had this problem along with another 391 owner. Both of us found that the j-trap connecter under the shower floor was cracked. It may have happened during manufacturing or just from the weight of a person in the shower stall. You can see the problem if you open the access hatch on the shower front and shine a flashlight under the shower floor. I'll add a picture of my cracked j-trap so you can see what it looks like.
Mine was repaired in a local RV shop by screwing out the floor drain and working the j-trap components out. I think it cost my about $100 and the mechanic showed me the cuts and scratches all over his arm from the struggle. The alternative would have been to pull the whole shower enclosure. I felt lucky.
Your configuration may be way different than the 391's setup but at least you have the benefit of my experience. Best of luck.
2007 391TS UFO Chassis