I have to tell you that I have installed over 75 of these drain kits on larger motor homes (as they almost all have interior drain lines) and more than a few Airstreams and I have yet to see one that does not work properly; that is to remove all of the condensate from the A/C and guide it to the drain cups, where it then flows thru the interior drain lines to the bottom of the coach. I have even installed them on a few stationary RVs with no drain lines, creating a drain path on the roof so the condensate does not run all over the place.
If your A/C is sloping to the rear of the trailer, you either have not tightened the gaskets sufficiently to compress them to 50% of the original thickness, or you have not positioned the included foam strips so they keep the rear of the A/C stable and level.
Yes, RVP Coleman units use a drain pump for the condensate, and these units are very comparable to the Dometic units. They are a bit more difficult to wire with a remote thermostat, as they require a 7-wire harness, not just a phone-type 4 conductor data cable.
PM me if you have any other questions.