hot water pressure relief leaking antifreeze?

Any time you drain your water heater 100% of the water doesn't come out, unless you roll the trailer on it's side or something. There's not usually this type of issue since most tanks are by passed and the little bit of water in the bottom has plenty of room to expand. I would prop the blow of valve stem in the open position and leave it. It's an aluminum tank you could take the drain out and put a heat gun on it to thaw out a little of the mixture and let it out of the tank.