You tighten the top unit to roof water seal by tightening the 4 bolts at the corners of the inside AC plate going upwards. These bolts compress the square hard foam gasket so that the condensate water stays on the outside roof and does not make its way inside to drip down. The hard foam gasget compresses some and starts leaking with age and road vibrations. These bolts should only be tightened to a medium tightness with an end wrench or socket. Overtightening can cause the gasket to fail prematurely.
With the age of your rig, it might be time for a new roof gasket, available on E-Bay.
To replace the gasket, you need to remove all 4 bolts, disconnect the connection electrical cable between the top and inside units, slide the top unit away from the roof opening, remove the old roof gasket, and re-assemble.
As with most any RV repair procedure there are videos on You Tube on how to do this.