I agree with the rest.
We always flood our black tank with gray water to flush it better. If we never used the toilet, I could easily see opening the right combination of valves so both tanks share the gray water you make.
As others mentioned, be sure to thoroughly clean out your black tank just in-case you later get a back-up into the shower pan. I clean our black tank annually with 1/3 gallon of bleach and fill with water to the toilet tube, let sit for an hour or two, drain and repeat twice without bleach to flush well. Bleach is hard on waste valves so you want to make sure you flushed it well.
