Never thought I would see one of these on a Street

It's not only that slides add weight. Because they require a "hole" in the walls of the living area structure, they compromize the strength of the structure - thus allowing joints to loosen over time.

I wouldn't want slides in any vehicle for use on rutted, bumpy, and tipped road surfaces. You want all the walls and framing in the coach box to remain rigid. That's also why good off-pavement camping vehicle designs isolate the coach box from the main chassis frame.