That rig is going to be working real real hard. Take it real easy and use the tranny to slow down on the downhill side of things. You are pushing the limits in my opinion. I would be more concerned with the tranny and brakes than the motor, all though it will be working hard!
In fact I think you may want/need a beefier truck for that trip. I re read and you plan to go through all the western states on a extended trip in and out of the mountains. I think you are bringing a knife to a gunfight. :( Your trailer is just a too heavy. JMHO