Some RV manufacturers are using a one-way check valve in place of a P trap in the drain lines. The valve is called a HepVO and is usually located where the P trap would be. It has a duckbill valve inside. Contrary to what you may hear, the valve can become clogged and stuck open. It can also get damaged if you ran a plumbers snake down the drain. You can remove them from the drain line, disassemble and check them out for cleaning or replacement. Be sure to reinstall pointing in the right direction!
