Freezing Temps And Tanks Not Draining

We moved out of our house and have to finish up a couple of things before leaving, and in comes the freezing temps to central KS. We are in a new to us, 2310 Hi Lo. My husband put a gallon of RV anti-freeze down both the gray and black water tanks. Two days later, with freezing temps, the tanks will not drain. The plunger in the shower brings up water and the pink anti-freeze. We didn't learn this until after dark, so will have to work on it tomorrow.

The temp last night was 11 degrees and the high today about 28, similar to this for yesterday. It will be 40 degrees tomorrow and 32 tomorrow night. 48 on Sunday. Central Kansas - a place we will flee as soon as possible.

We have Buddy Heater, the big one in storage that we can access. I have searched here and googled for ideas. I wish I could better pinpoint where it may be frozen, it is not the thingy that you pull open as that is fine. I am thinking maybe some water went into the tank, formed an ice cube near the outlet and it is blocking the whole deal.

Any thoughts or advice would be appreciated. Yes, I know this may seem less than bright, but sold the house and the weather decided to be a good 20 degrees colder than last year. This is what we have to work with.

Thanks!