There are different types of leaks and they have different levels of severity...
It could leak in such a way that every time water flows into the sink and to the waste tank you get a leak. This would be the worst kind as it is constant. For example, imagine a pipe that running at an angle (or horizontal) and you have a leak on the bottom side of the pipe....anytime water passes by it will leak.
It could be a leak such that it will only leak if the water level goes above the point where the leak is. For example, imagine a pipe that is running at an angle (or horizontal) and you have a leak on the top side of the pipe....when water runs by it likely won't leak. But if/when the tank (and this pipe) get full then it will leak. This would be a less severe/urgent issue. However, if the leak/crack/hole is big enough you might start getting smells.
One way to test your leak... Start with a clean black waste tank (use your tank flush if you have one). Fill it with water (and a little bleach in case you do get some leaking) until it is 100% full. In theory, you should be able to keep adding water to your toilet until you can see water filling the pipe from your toilet to your waste tank, and then even further until you literally have some water starting to fill your toilet bowl and not going down. At this point, your black waste tank is 100%+ full and the lines from your toilet and also from your bathroom sink will contain standing water. This system is designed to be water tight so if there are no leaks the water will just sit there in your toilet bowl at the same level for hours and hours. If it slowly drains lower, you know you have a leak somewhere.
Note: I've read that some bathroom sinks in some RVs/trailers run to the gray water tank and not the black tank. All of mine run to the black tank.
Anyways...I hope your leak stays plugged. These RVs/trailers are supposed to be about enjoyment...not as much fun when we have to work on repairs.
Happy Camping!
Chris
