18,000 on my 12 2500 CTD. No issues yet. I do some in town putting around as well as some shorter 55-60 miles runs of 2-3 miles. I try and tow at least once a month.
When towing in the mtns I run in 5th most the time as the speeds vary and 5th puts it at 1900+rpms. Problem with leaving it in 6th for the mtns is at 55mph the rpms are about 1400. 200 under peak TQ. Out on the freeway I'll run 65mph if I can.
Always run the EB in town and keep it in 4th. I never just drive it in drive around town. Out towing I drive pretty hard. I always try and maintain speed up hills/mtns. The motor likes it and so do I so it's a win win situation.
Mian thing is to keep the rpms up a bit. Helps it warm up faster and keeps the tranny form shifting all day.