Originally Posted by nmbosa
.. it looks like the drain pan connection is about 6 inches back from the front of the 14" opening on the street side of the trailer..
"About", in your description, appears to be the issue. I agree - it's all about tolerances.
Look at it another way. The future AC requires a 14 inch square hole for operation. BUT, the actual unit will cover much more area. Although I will never be in your situation, my recommendation is to run the condensate line to the general location, but drill no through-holes. If/when you get an AC, then drill a hole top-side to accomodate it, then go inside and tin-snip the interior skin & connect the drip line. My bet is that the inside portion of the AC will cover the tin-snipped hole
There's plenty of time now to determine if this is a good approach.
Also, a 1/2-inch diameter drip line is my recommendation. While a considerably smaller line is all that's necessary for the flow rate, smaller lines clog more easily.