[JRTJH]

You should have two sewer hoses connected to your trailer. So far, I haven't seen any discussion or confirmation from you that you're aware of the two locations to drain tanks. One is forward of the axles (galley gray tank) and one is aft of the axles (bath gray and black tanks). Please confirm that you have ALL THREE valves open and are aware of their location. That's one of the most common mistakes made by new owners, not knowing where to dump the galley tank.

The Springdale 270LE does not have "freeze protected waste tank valves"... The three valves are under the trailer and exposed to the elements (see photo below). This is both good (maintenance) and bad (freeze protection).

It should be relatively easy to verify if water is coming out of the front gray tank by removing the sewer hose and verifying whether the slide valve on the gray tank valve is opening or not. If it is opening, then insert a flexible rod/spring/snake (BE VERY CAREFUL NOT TO PUSH TOO HARD) and insert that rod into the gray tank. If you meet resistance, then you'll need to determine why. If the rod goes into the tank, then it's apparent the "stoppage" is in the drain line from the sink to the tank.

In this photo you can clearly see the galley gray tank valve, just forward of the axles. It is in the open position (see the handle on the rod, pulled about 7" forward). Your trailer has the same valve in the same location. If it doesn't, then it's possible, although I'd find it hard to believe, that the factory failed to plumb the drain line from the galley gray tank to the sewer outlet.