Washer drain not tied into grey water tank??

[jfaberna]

On my 2014 3402RL I've been trying to better understand the plumbing system and re-winterize it to assure myself the RV tech did it right at PDI. I'm sure he did but I couldn't observe all the steps he did because he was fast. Not his first winterizing

I hooked my 30 PSI regulated compressor to the black water flush first, and then hooked it to the fresh water connection. I blew each faucet hot and cold separately and went on to the next. I added antifreeze to the traps as well. Lastly, I ran the normal cycle on the washer for a couple of minutes to assure the air was coming into the washer via both hot and cold connections. I canceled that, then put a quart of RV antifreeze in the drum and ran a drain and spin cycle.

Well everything went as planned except I had antifreeze coming out the sewer connection when I removed the cap to check it. I was puzzled since both grey and black water valves were shut. No harm done to the environment because I've never had anything in the tanks but fresh water during PDI. So I looked under the Monty and saw that the 4" drain comes out of a compartment to the sewer cap but has 2 other 1 1/2" lines attached before the cap. One seems to be the vent on the roof and the other seems to be the washer drain or at least it in the same area. Does this make sense?? I can see the logic because washing uses a lot of water and dumping it to the holding tank could cause overflows.