Thank you, Lewster!! I found this thread very helpful when I installed a new Dometic 15K AC/heatpump (Model 651816) on my 2009 FC 25FB. The old AC did have the big pan under it, and it was an ordeal getting it and the factory installed caulking off the trailer.
On the new install, rather than cut completely thru the gasket to accomodate the drain cup tubing, I used a 5/8" forstner bit. This type of bit cuts a clean hole just big enough to slide the drain cup tubing thru.
I did have a problem with one mounting bolt spinning freely. The bolt went into the nut part way, wouldn't tighten further, and couldn't be backed out. This led me to remove the Dometic's shroud, styrofoam cover, and inner plastic housing to access the nut. The nuts were 'glued' to the Dometic's base with caulk, but the one nut was missed, allowing it to spin freely. I was able to secure it enough to tighten the bolt.
I did not tighten the mounting bolts enough to reduce the gasket thickness from 2" to 1". The foam gasket appeared to be a closed cell type that does not easily compress. I torqued those bolts well, and no leaks.
Here's a couple photos showing my drain line connection and a gap around the wiring port in the Dometic unit's inner plastic housing--this latter gap allows insects to get into the trailer interior from outside.
Drain line connection--two layers of gasket material visible:
Caulking gap at wiring port of inner housing--brightly lit part of photo is light from trailer interior:
Good luck to everyone doing an AC R&R themselves!!