I have a 23' Roo as well. My pump worked fine for the first 3 seasons but at the end of last season the pump started "burping" about every three minutes. Like the OP I search for a leak from one end to another and could not find any sign of water. Last week I replaced the check-valve and was thrilled when the "burping" stopped. We then went camping for a week and on the 5th day it started again.
It has to be the check-valve because if there was a leak replacing it would have had no affect. I'm extremely frustrated that the "burping" is back and even though the check-valve is not expensive and a fairly easy repair, I'm not about to do it every week.
The next step might be to replace the pump outright.