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Most are secured to just the ceiling, but it is usually reinforced underneath. The solution is usually to jack them back up into place and use some larger diameter screws and perhaps add some L-brackets with several more. This seems to be more prevelent on older Fleetwood units for some reason. Hope that proves helpful here. Please get back to me if you need more on this as all follow ups are included and expected.
It might be helpful here for you to attach a photo or two of the cabinets so I can make some further suggestions here.