There's so much bacteria in waste that holding tank contents don't even notice whether flush water is salt or fresh...so no need for any concern, whether it's for a day or a year.
It's only in the anaerobic environment in toilet discharge hoses that it matters because sea water can generate odors that can permeate the hose, fresh water shouldn't (but sometimes does).
© 2016 Peggie Hall
Specializing in marine sanitation since '87.
Author"The NEW Get Rid of Boat Odors"