When to change oil?

I tried doing a search on oil changes but no results were returned. My apologies if this subject has been previously discussed.

2015 F250 6.7L... With these newer computer controlled engines, the computer wants to tell you when to change the oil, but that may not be good enough for me.

A little background first. I use my truck mostly in the summer, pulling a fifth wheel trailer, and moving from place to place, out here in the West. I am frequently in locations where there are no Ford dealers or otherwise competent diesel oil change businesses. Therefore I need to plan my oil changes, if I can. 50% of our miles are towing.

Last summer I changed to oil in the middle of Colorado at a dealer, even though I had only 5000 miles since the last oil change, with the computer telling me that an oil change is not needed. I then drove straight to Yellowstone, to spend a week, with plans to drive straight to the Southern Oregon Coast afterwards. This was almost 2000 miles with no opportunity to stop for half a day to get an oil change, let alone finding a place to drop the trailer first.

If I can predict when I'll need an oil change, then I can plan for a down day to get the oil changed. Knowing how the computer determines when the oil needs changing would be nice. Does it do it strictly on miles driven, a combination of towing miles and non-towing miles, a measuring of the dirt in the oil, etc? Then once you get the alert about the "oil change needed", how far can you drive (towing?) before damage might be done to the engine? Does driving on unpaved roads affect the number of miles between oil changes?

Right now I've got about 7000 miles since the last oil change, so I'm expecting the alert to come soon. We have a short, 350 mile towing trip coming up in 2 weeks, maybe the alert will come then.

Thanks.