Depending on the age of your unit there are 4 or 6 screws holding the ceiling unit up to the ceiling.
Unlatch the black plastic screws by turning them 1/4 turn and drop down the center filter panel.
This will expose the mounting screws which again depending on the age of your unit may or may not be spring loaded. If they are spring loaded there will be 2 extra screws one on each side.
As you drop the plenium down be careful of the wiring
There are 4 8" long bolts that hold the upper A/C unit to the roof via a metal bracket under the ceiling.
We always use a cordless screw driver with a clutch in it to insure that we torque each bolt evenly in a cross corner pattern. You can tighten these bolts pretty darn tight
This should solve your leak if it does not you may have to remove the upper unit and replace the gasket which should be available from any good RV shop
Total time required about an hour