Yep - 5 gallon bucket will drain it into the bucket, then clear the way to the bathroom and flush it down the toilet.
It really does not smell that bad. . . You can add some black water treatment the day before, and that should help with any smells. Also because it is less than 40F it should not smell that much anyway. It is only when above 70F that it starts to smell bad.
I know someone who had to dump their black water tank into a outhouse along the Alaskan Highway. Dump stations where not at that section of roadway, and she was really full! It really was not all that bad.
And you are dumping mostly water anyway.
Diluting the antifreeze by 5:1 will change it's ability to resist freezing from -20F to something much warmer, probably close to 20F? But that is a pure guess, it might damage the plumbing, or might not? Why chance it?
Fred.