Wow. That is close. I agree with you that it is likely you would get some problems. My black tank vent on my pop up is on the other side of the roof. Not sure what you can do to fix the problem other than finding a new way to route the pipe.
I have made an extension for the black tank vent. I'll install it this weekend and see what happens. The smell is either from the vent or the turlet. It's definitely not from me...lol thx for the inputs.