If you have the Dometic auxiliary drip kit # 3107688.016, then follow this procedure.
You do indeed have to cut a gap in the sealing gasket that is installed by the factory on the bottom of the unit. Install the cups first (you probably have to remove a 1/4" hex head screw near or underneath each the mounting location of each of the cups. Use a couple of TEK (drill point) screws to mount the cups to the underside of the A/C base pan centered on the drain spout.
Next, lay the drain hose on top of the gasket and trial fit it to the flanged spout so you know exactly where to cut the gasket.
Now remove the hose and cut the gasket right down to the base pan and remove the piece.
Install the drain tubing onto both drain fittings and be sure to attach the clamp BEFORE
connecting to the fitting so it is already on the hose when you attach the hose to the fitting. Then move the clamp over the hose and fitting and it will make a tight seal.
fill in the gap you cut (a little over fill is fine) with Vulkem, SikaFlex 221 or any other good sealant like DAP. DO NOT USE SILICONE!!!
Be sure to get your sealant under the hose as well by lifting it slightly and then re-seating it in the sealant. The sealant fills in the gap and completes the water tight seal.
The final step is to add the second gasket right over the original so that you now have a double layer of gaskets. It should be self stick...just peel off the paper and apply. It should be identical to the original.
You are now ready to install the unit on your roof. Tighten the 4 bolts from the underside so that the gaskets compress approx. 50%, connect the drain hose from inside the A/S and you are ready to install the air distribution box from underneath.