I've had my 2010 Mercruy Mariner now since 2009. I've towed it 4 down for about 30,000 miles since then with no issues. But I am a fanatic about the procedures and TSB's associated with it.
The TSB is #12-11-3. When I took my Mariner in 2 months back for a routine tranny fluid flush, I printed that out for the techs, then checked it myself to be sure I was happy with the level.
While the owners manual says to warm for 5 mins every 6 hours, I do it every 3 to 4, and hold the tranny in gears about 30 seconds before finally turning it off.
So at 55,000 miles driving and 30,000 towing, I'm still pleased.