As suggested......tighten the 4 hold down bolts------just 'snug'
You have to remove the whole ceiling grille. Usually 6 screws. 1 on each corner and then 2 in cross bar under air filter cover.
The 4 hold down bolts are in the corners.
They secure the whole A/C unit to the roof and compress the roof gasket.
They can get loose.
The roof gasket is 1" thick when new and gets compressed to 1/2" to seal.
Over-compression can warp the gasket and also block off the evap coil drip pan drain holes.......pan ends up to close to roof and the drip pan can't drain condensation from evap coil (which can cause the built up water to get carried over by blower and leak inside rig from ceiling grille when you run it come summer.)
Check the bolts...snug them up.
If it continues to leak rain water then a new gasket will be needed.
You have to disconnect electrical to the A/C unit (should be able to do that thru opening where air filter is.
Undo the 4 hold down bolts and then lift the whole A/C unit up off/out of roof opening. Remove the old gasket, install new, set A/C back into roof opening (check that gasket is properly aligned)
Install and tighten hold down bolts (snug)...connect electrical wiring.
Then check hold down bolts again after a couple of trips.
Good Luck!......Hope it's just loose bolts