I have done part of that route. I'd go to Trinidad, CO, then to Amarillo, Childress, Cisco and Austin.
The drive from Trinidad to Amarillo was a little slow with the MS75D, but not too bad. Mostly downhill going west to east. Plan to eat a meal in Trinidad while you top up. There is a "Green Grocer" next to the SC there, too, if you're into that.