Should be fine IMO. Suggest adding 1 gallon of RV AF to the tank now, and have one on hand as Frank said. You could poop a few times too, without risking damage to the black tank IMO. For a short trip, no need to add water and chemicals, although there could be some odor, when you open the trap door to release the contents of the toilet. When you get home and can dump, you can always stick a hose down the toilet to flush out any solid material left.
If the weather is mild, fill the black tank 1/2 full with water [from a hose down the toilet] as you get close to home, and let the road action macerate the contents into a slurry [if you added solid human waste material during the trip].
Damage from liquids freezing only happens in a mostly-closed container, with vertical sides. An almost-empty black tank with a small mound of semi-solid matter in the middle, surrounded by RV AF mixed with urine, will not do any damage IMO.
Within moderation that is!