Where did you put the wye? Of the people I have known to have redone the plumbing to a single dump point none have seemed to have a problem.
Can you post a picture of how your exterior plumbing looks now? Where are the gate valves? did they move them?
If the gate valves are still where the factory put them, and they are closed, and the "full-burp-not" thing is going on right at the "input area".. then my guess would be you have a bit of a partial obstruction just below that point, where the pipe goes into the holding tank.. Only one way to know for sure, and of course that is the only way to clear it..
Welcome to RV plumbing fun 101.
If that is indeed the case, after clearing, use more water when flushing, and eat more fiber.