Interstates are not really all that steep compared to some secondary roads, so you shouldn't have any problems maintaining a safe speed while climbing. Your Axis has far more power for its weight than loaded-down trucks so you'll be passing many of them while climbing.
I've driven across the Rocky Mountains many times in different E-Series motorhomes roughly the same weight as yours, all with same Ford 6.8L V10, and it was never an issue at all.
First, I stay to the right so I don't interfere with faster car traffic (unless passing slow trucks). I also take motorhome out of cruise control so I can control speed and power directly.
I prefer to use the tachometer to keep engine around 3,500 RPMs. For motorhomes in 12,000 to 14,000 pound range that yields adequate speed going up 6 to 7 percent grades. I just downshift as needed until I can hold +/- 3,500 RPMs at about 75% throttle. I don't ever floor it because I'm on vacation and not in that big a hurry. As long as I'm not holding up traffic what's an extra minute or two?
Going down on Interstates usually isn't that difficult because wind drag helps a lot at highway speeds. On the other hand curvy secondary roads can make it tough to find the right gear to provide enough engine braking.
Sometimes I've had the engine go up to 5,000 RPMs due to engine braking, which was too high to downshift -- yet it wasn't enough to slow motorhome enough for road conditions. In that case I use the brakes to slow quickly (so as not to overheat them) and then shift to a lower gear. If that lower gear isn't enough to provide enough engine braking, I then apply brakes again until I can shift to next lower gear, etc.
In my experience with similar size motorhomes Interstate highways are fairly easy to drive on. The only time I had a problem with overheating was climbing out of Death Valley going east (not an Interstate). On a loaded E-450, speed was only in the 20 to 40 MPH range, making it difficult to find the right RPM and gear combination. That was with older 5-speed where 1st wasn't fast enough and 2nd was too tall. With newer 6-speed transmission, finding the right gear should be easier (I hope).