Hi,
If your gasket is leaking, then time to replace it. If not leaking, leave well enough alone, and just install fender washers under all the bolts, one at a time, and it should be fine. Any leaks will just get the inside a little damp, and drip through the inside cover to the floor.
To replace the gasket, you need to disconnect the 120 and 12 volt wires to the unit, place a thick blanket on the roof, and then loosen all the bolts. Move it over to the blanket (to protect the roof) and then replace the gasket. Transfer everything back to the top of the new gasket, and put back the wires / bolts.
If you do not feel up to the task of moving everything, then for about $200 your local RV dealership can do it for you. The A/C is about 95 - 120 pounds. . . Yes it would take two people. .
Good luck,
Fred.