OP- I wonder if tilting your trailer more or less towards the black tank would make a difference?
Might be an interesting test. If it does make a difference, you know that changing your pipe slope will help. If not, you just saved yourself a whole lot of hassle ;)
But- it seems you shouldn't have a problem there no matter what. Is the section from the toilet to the 90 elbow enclosed, or is it something you can move? Might be able to get a more direct route, or use 45's instead of a 90?
We haven't seen a pic yet, which would be a good idea (and easier) than tearing into it. Sometimes what we picture from descriptions isn't exactly what you have ;)
