Can you try fitting something like a soccer ball under them? Anything along that line will of course have to be removed before closing the slide, but, it would support the center.
I am facing this and have never had them before. Your complaint is common one though.
Charles & Martha Myatt
2009 Duramax w/Transfer Flow Fuel Tank
2016 Flagstaff V-Lite 30WFKSS
Almost South of the Border