You could do the line pinch and narrow it down, if you just pinch the water line that goes to the shaft seal before pulling it out of the water it would eliminate that line and any water coming out of there would be bilge water. You could also just leave your plug in on the trailer and fill your bilge with water, if it starts leaking as soon as the level gets even with the shaft seal it would be an easy sell to the dealer. I'm sure they don't want to do the work...shaft seal replacement sucks and it comes with an engine alignment that also sucks....pretty time consuming job.
My best friend lives in Durango, great town.