The cabnets are screwed to the metal framing of the trailer. Inside the top of the cabnet there is a panel stapled to the wood framing of the cabnet. Pull out the staples and remove the panel and then rescrew the cabnet . You probley will find the orignal screw has broken and should be removed. I usuly add several extra screws while I am there to prevent repeting the repair. Reatach the paneling by restapling it to the wood strips. This repair should not take more than a hour to complete.
Sam
I your cabnet does not have a panel it should be easy to see the wood frame (2x2). Just screw the framing to the roof there should be a metal beam there . Be sure you use self taping screws or you will have to pre drill first then install the screws to hold the cabnet to the roof.
Sam