Your question depends on what parts of I25 and I70 that you plan on driving. I25 does not have much except for the grades up and down Raton Pass at the south end of Colorado. Raton Pass is not a difficult pass.
I70 East of Denver is mostly flat.
I70 West of Denver has many grades of about 6%. There are relatively few curves so the car traffic travels 65-80 MPH depending on the driver. Be careful that you don't allow your speed to climb to match the cars. You don't have the stopping power of a car. Use the engine as a brake. If the tow haul mode doesnot keep you in a low enough gear to hold your speed on the downhill then you need to downshift manually.
As mountain driving goes, I70 isn't too bad, just use your engine for a brake and go slow.
'88 Bounder on a P30, Gear Vendors, Thorley and other goodies.