My Xtrail would never do a DPF clean when towing, but often did one a day or so after arriving at our destination, often with a DPF warning light. The thoughts were on the forum that if the engine was hot, the DPF clean was inhibited to prevent getting too hot. My Outlander did a big tow to Scotland this year, plus lots of slow driving around when there, then a big tow back. A week or so when home I was aware the the oil level had gone up to the X level (overfull) and had to go to the dealer for a forced re-gen and oil change (£240). It had clocked up a number of failed passive re-gens but no DPF warning light. My conclusion is when towing they don't re-gen.Over the past 3 months we have done a couple of long trips towing the caravan so assumed that this would have cleaned the DPF. I am not a mechanic so unknown territory to me.
Top of my diesel wish list is a dashboard light saying "DPF nearly full, give it a blast!"