Well, if the hill country gave you trouble, wait for the Rockies. I'm assuming you're coming from FL (St. Petersburg) and you've made the trip to TX before.
Other than the hill country, TX is non eventful. If you're going thru Las Cruces on 25 there are some hills around there. It's been a long time since I used 25 going there but there is quite a long, tall grade coming in on hwy. 70 as you cross the Organ mountains; I think the same holds true coming in on I25; someone will probably expound.
Raton Pass used to be much worse than it is today but is worse than most hill country roads grade wise. It is a divided 4 lane and very wide.
Other than that you've picked a route that is pretty tame with not much scenery - pretty forgettable really. You are going to be traveling through some areas that have some awesome things to see and do. It's a shame to just drive thru it and stay on the freeway. Royal Gorge, Garden of the Gods, Pike's Peak just flipped in my mind. Also, at Ft. Collins you could cut off and go to Walden (moose capital of the U.S.??). Beautiful drive following a river I can't remember the name of. At Walden you can cut back NW and get back to I80. It IS slow, windy and somewhat hilly - and 2 lane, but beats the heck out of the big highways.
The 5.7 is going to labor on some of the grades. I would double check your truck and trailer brakes; you will have a box weighing right at 10,000 lbs. trying to shove you down those mountains....or pull you over the edge
Having pulled about the same load with about the same truck I will tell you that you need LT tires on the truck; air bags in the springs and a GOOD anti sway/WDH setup. Use tow/haul along with gear selection going downhill.
What kind of brake controller do you have? Make sure it is set properly and you know how to use it. Take is slow and take your time - enjoy what there is to see. Without a larger truck I'm not sure you will be able to "enjoy" what's out there because depending on the circumstances that big of a trailer behind that truck CAN become a handful. Just be careful.