Going cross country, I have found I40 to be pretty level driving coast to coast (a few hills in AZ and NM). I80 in WY has some grades west of Laramie. Of course, I70 west of Denver is very mountainous. I90 through MT isn't so bad, but I have never been on 90 west of Bozeman MT. If the southern route is your thing, I10 is flat as a pancake over most of its length.
WRT lengths, it seems the recreation.gov and reserveamerica.com allowed lengths have some safety factors built in so a few extra feet over will not cause problems in my experience. I always took the length allowed to be the length of the rig alone minus the tow vehicle.
