I haven't checked this site for a couple of days. Sorry to have a late reply to your post. I recently hauled my 16 ft Lund about 900 miles with a Toyota Highlander, and had an unusual problem.
After I arrived, I thought I would replace the gear oil in the lower unit. When I took the screw out, there was no oil in the lower unit. It was completely dry. If I had gone fishing, I would have ruined the lower unit.
I had the unit pressure tested, and it was fine - no leak. My best guess is that when the marina serviced my boat, they reused the old washer (gasket?) instead of using a new one. With an imperfect seal and 900 miles of bouncing on the trailer, the gear oil very slowly leaked out.
I will never again put my boat in the water after a long trip without checking the lower unit.