I don't have a number, but I doubt if it's something you need to worry about. The RV sofa you now have is no lightweight, and anything that fits in the same space probably won't be much different in weight. The slides are weak or lacking in support, so anything short of stacking bricks in that space ought to be fine.
You could call Fleetwood Customer Support and ask, but I doubt if they've ever come up with a specific weight limit either.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL