I travel to/from BC and Denver quite often (Sierra, 6.2L) sometimes towing a smallish (under 4000 ibs) trailer and sometimes, not. I travel the major highways and, depending on whether the trailer is ...
The transmission is controlled by your computer box new) or by your vacuum level (old), to make the transmission efficient and not over-stress it.
You could easily use the tow feature which locks out the highest gear, thereby keeping the RPM higher, but if you still try to cruise at high speed in this way, you will over-stress the trans.