Based on your description, it sounds like your black tank valve is closed (good!), so it's not coming up through the actual sewer.
I'd have to agree with the others that said exhaust fan. If you have some windows open in your trailer and the wind is blowing just right(wrong), that could potentially cause the similar effect of having slightly negative air pressure in your trailer, so the black tank air comes up through the toilet valve when open.
If you replaced your vent cap with something like this
It would help make sure that the negative air pressure is inside the tank and therefore air goes down into the toilet instead of up.
We usually have a roof vent and a window cracked open for condensation/fresh air reasons, and the only time I noticed it in our old trailer was when the bathroom exhaust fan was on. I got in a habit of turning it off, flushing, then turning it back on.