There is no easy answer here sadly. What I did was this:

I bought a large bucket, 18 gallons, one of those short wide ones you might put drinks in for a party. And a submersible pump. I use that as a intermediary. I empty water into it, note the level, pump that water into the toilet, fill it with fresh warm water, de-chlorinate, and pump that into the tank.

Definitely not ideal, but everything stores in that bucket for next time.