I had a similar problem with my unit when I first got it. I discovered that the lag screws that they use to hold the AC units on the roof are the problem. The screws are driven through the roof and into the top of the ductwork. I bought a few tubes of self leveling roof sealant and sealed the around the four AC roof fasteners and the leak was solved. Squirt the caulking under the upper side of each bracket to cause the water to go around the bracket and not under it. The more stuff you do yourself the better you will understand your unit and you will save money and keep your unit out of the repair shop.
2018 Redwood 3901WB pulled by 2016 Chev 3500 HD DRW. Goodyear G114 tires, Moryde I.S., Kodiak Disc Brakes, Splendide W/D, Tire Minder TPMS, Sleep # King.