The answer to your question is in the FAQ.

Normally, the interval given in the owners manual is reasonable, though often on the looser end.  

However, for a heavy duty truck like that there might not really be a standard interval that is appropriate for everyone.  To get around this, the manufacturer often provides 2 maintenance schedules in the manual: one for "normal conditions" and one for "severe conditions" (towing, off-road, etc). And sometimes, they don't give a change interval for normal conditions, only for severe duty.  For example, the manual might say change it every 30,000 miles for severe duty and only "inspect" it every 50,000 for normal duty.

If they give a severe duty change interval, go ahead and follow that, especially if you are doing any towing at all.  If they don't, you can get a number of opinions here in the FAQ and search bar.  If you are still in doubt, change it every 30-50K.