First I'm not an RV repair guy, but I do build and install cabinets in houses, shops, etc.
Identify where the screws are going in. Use a "good" stud finder and see if you can identify any locations good for screws. It may not be easy but new screws into the ceiling - or sides may help. If you do this, try to raise the cabinet with a jack or 2x4 pushed up FIRST so the cabinet is in the correct location. Do drill pilot holes and countersink screws if possible just to look neat. Don't use longer screws as you may penetrate something you don't want to, like roof or side exterior.
Or - get cabinet into position and use the next larger size screws. So if you take out some no. 8's, put in some 10's. Use same hole but better bite.
Also, if there is a location of wood to a solid surface _not vinyl- you may also use good construction adhesive WITH the screws, but not just adhesive.
Good luck. I know this is simple but hope it offers something.
John and Caren, Arkansas. 2006 Newmar Cypress 5th wheel. "Fractured Fairy tails"