I have the same issue.
I purchased a sewer cap that accepts a water hose.
The end of my sewer hose accepts the other end of the water hose. (see attachment)
I purchased a grey water hose that I only use for this purpose.
I make all of the connections and open the galley valve before I open the slideout.
I have been doing this for at least three years and it works great.
2009 Flagstaff 8528 RKSS
2007 Toyota Tundra DC 5.7L V8
Pullrite Superglide 14K
Air Lift 5000
Prodigy Brake Controller
"I've got a mind like a steele.. a......a steele......um.....uh...... what's that thing called again?