I would not worry about it-if the lines had water in them, when they ran the RV antifreeze through, that removed any that may have been left.
However, if there is no RV antifreeze in the toilet, I would drop a cup in there. You want to keep that seal from drying out.
Heck, just so I wouldn't have to worry about the antifreeze freezing over winter, I almost thought about draining it after running it through. I didn't bother though. Someone told me if the antifreeze freezes, it just crystallizes and doesn't expand...don't know how much truth is in that, but works for me. :-)
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