Originally Posted by clutch
The vent on our pull trailer dosen't go up through the roof. It is a one way valve under the sink. If you have the same thing you can turn it or tap it and the one way valve may work better.
If the valve you are talking about is at the top of the vertical drain line by the sink, it is an Air Admitance Valve (AAV). It's purpose is to admit air in the drain line when water is flowing to avoid syphoning water from the drain traps connected to the line. Typically, you will have one for the bathroom sink and one for the kitchen sink. The shower will probably drain into the same drain line as the bathroom sink and will be served by the same AAV.
Another one way valve that has been popular in Europe for years but is only just starting to be used on this side of the pond is a check valve that is installed horizontally between the sink and the vertical drain line. It functions as both a trap and an AAV. Unlike AAVs, the position of those is critical. lI haven't seen any in the 2011 Jaycos I've looked at but I haven't had a chance to look at any 2012s yet.
I would be surprised if your TT didn't have a roof vent for each of the holding tanks (they also can be combined into one roof vent). Depending on AAVs at the sinks and shower would be inadequate because if the gray water tank developed any pressure, it would have no where to go since an AAV only admits air, not releases it (and you sure wouldn't want to release the wonderful aroma lurking in your gray water tank into your living space!). The black water tank also needs a roof vent to release any pressure build up and draw in air when the toilet is flushed. Without a vent, any pressure build up in the tank would vent out through the toilet when flushed, not exactly a pleasant thing. When dumping the tank, air has to be admitted into the tank to allow it to flow or the tank might collapse and the water flow would be too slow to properly wash away the solids. Also, without a vent, the toilet would gurgle when flushed.
In the case of the OP, I suspect either the roof vent for the tank is blocked (bird's nest?) or the vent pipe has dropped down into the tank. Those are the most common causes of gurgling.